# Shortest Path Faster Algorithm

• Difficulty Level : Medium
• Last Updated : 25 Nov, 2021

Prerequisites: Bellman-Ford Algorithm
Given a directed weighted graph with V vertices, E edges and a source vertex S. The task is to find the shortest path from the source vertex to all other vertices in the given graph.
Example:

Input: V = 5, S = 1, arr = {{1, 2, 1}, {2, 3, 7}, {2, 4, -2}, {1, 3, 8}, {1, 4, 9}, {3, 4, 3}, {2, 5, 3}, {4, 5, -3}}
Output:
1, 0
2, 1
3, 8
4, -1
5, -4
Explanation: For the given input, the shortest path from 1 to 1 is 0, 1 to 2 is 1 and so on.
Input: V = 5, S = 1, arr = {{1, 2, -1}, {1, 3, 4}, {2, 3, 3}, {2, 4, 2}, {2, 5, 2}, {4, 3, 5}, {4, 2, 1}, {5, 4, 3}}
Output:
1, 0
2, -1
3, 2
4, 1
5, 1

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

In case you wish to attend live classes with experts, please refer DSA Live Classes for Working Professionals and Competitive Programming Live for Students.

Approach: The shortest path faster algorithm is based on Bellman-Ford algorithm where every vertex is used to relax its adjacent vertices but in SPF algorithm, a queue of vertices is maintained and a vertex is added to the queue only if that vertex is relaxed. This process repeats until no more vertex can be relaxed.
The following steps can be performed to compute the result:

1. Create an array d[] to store the shortest distance of all vertex from the source vertex. Initialize this array by infinity except for d[S] = 0 where S is the source vertex.
2. Create a queue Q and push starting source vertex in it.
• while Queue is not empty, do the following for each edge(u, v) in the graph
• If d[v] > d[u] + weight of edge(u, v)
• d[v] = d[u] + weight of edge(u, v)
• If vertex v is not present in Queue, then push the vertex v into the Queue.

Note: The term relaxation means updating the cost of all vertices connected to a vertex v if those costs would be improved by including the path via vertex v. This can be understood better from an analogy between the estimate of the shortest path and the length of a helical tension spring, which is not designed for compression. Initially, the cost of the shortest path is an overestimate, likened to a stretched-out spring. As shorter paths are found, the estimated cost is lowered, and the spring is relaxed. Eventually, the shortest path, if one exists, is found and the spring has been relaxed to its resting length.
Below is the implementation of the above approach:

## C++

 `// C++ implementation of SPFA` `#include ``using` `namespace` `std;` `// Graph is stored as vector of vector of pairs``// first element of pair store vertex``// second element of pair store weight``vector >> graph;` `// Function to add edges in the graph``// connecting a pair of vertex(frm) and weight``// to another vertex(to) in graph``void` `addEdge(``int` `frm, ``int` `to, ``int` `weight)``{` `    ``graph[frm].push_back({ to, weight });``}` `// Function to print shortest distance from source``void` `print_distance(``int` `d[], ``int` `V)``{``    ``cout << ``"Vertex \t\t Distance"``         ``<< ``" from source"` `<< endl;` `    ``for` `(``int` `i = 1; i <= V; i++) {``        ``cout << i << ``"             "` `<< d[i] << ``'\n'``;``    ``}``}` `// Function to compute the SPF algorithm``void` `shortestPathFaster(``int` `S, ``int` `V)``{``    ``// Create array d to store shortest distance``    ``int` `d[V + 1];` `    ``// Boolean array to check if vertex``    ``// is present in queue or not``    ``bool` `inQueue[V + 1] = { ``false` `};` `    ``// Initialize the distance from source to``    ``// other vertex as INT_MAX(infinite)``    ``for` `(``int` `i = 0; i <= V; i++) {``        ``d[i] = INT_MAX;``    ``}``    ``d[S] = 0;` `    ``queue<``int``> q;``    ``q.push(S);``    ``inQueue[S] = ``true``;` `    ``while` `(!q.empty()) {` `        ``// Take the front vertex from Queue``        ``int` `u = q.front();``        ``q.pop();``        ``inQueue[u] = ``false``;` `        ``// Relaxing all the adjacent edges of``        ``// vertex taken from the Queue``        ``for` `(``int` `i = 0; i < graph[u].size(); i++) {` `            ``int` `v = graph[u][i].first;``            ``int` `weight = graph[u][i].second;` `            ``if` `(d[v] > d[u] + weight) {``                ``d[v] = d[u] + weight;` `                ``// Check if vertex v is in Queue or not``                ``// if not then push it into the Queue``                ``if` `(!inQueue[v]) {``                    ``q.push(v);``                    ``inQueue[v] = ``true``;``                ``}``            ``}``        ``}``    ``}` `    ``// Print the result``    ``print_distance(d, V);``}` `// Driver code``int` `main()``{``    ``int` `V = 5;``    ``int` `S = 1;` `      ``graph = vector>> (V+1);``    ``// Connect vertex a to b with weight w``    ``// addEdge(a, b, w)` `    ``addEdge(1, 2, 1);``    ``addEdge(2, 3, 7);``    ``addEdge(2, 4, -2);``    ``addEdge(1, 3, 8);``    ``addEdge(1, 4, 9);``    ``addEdge(3, 4, 3);``    ``addEdge(2, 5, 3);``    ``addEdge(4, 5, -3);` `    ``// Calling shortestPathFaster function``    ``shortestPathFaster(S, V);` `    ``return` `0;``}`

## Java

 `// Java implementation of SPFA``import` `java.util.*;` `class` `GFG``{``    ``static` `class` `pair``    ``{``        ``int` `first, second;``        ``public` `pair(``int` `first, ``int` `second)``        ``{``            ``this``.first = first;``            ``this``.second = second;``        ``}``    ``}``    ` `// Graph is stored as vector of vector of pairs``// first element of pair store vertex``// second element of pair store weight``static` `Vector []graph = ``new` `Vector[``100000``];` `// Function to add edges in the graph``// connecting a pair of vertex(frm) and weight``// to another vertex(to) in graph``static` `void` `addEdge(``int` `frm, ``int` `to, ``int` `weight)``{` `    ``graph[frm].add(``new` `pair( to, weight ));``}` `// Function to print shortest distance from source``static` `void` `print_distance(``int` `d[], ``int` `V)``{``    ``System.out.print(``"Vertex \t\t Distance"``        ``+ ``" from source"` `+``"\n"``);` `    ``for` `(``int` `i = ``1``; i <= V; i++)``    ``{``        ``System.out.printf(``"%d \t\t %d\n"``, i, d[i]);``    ``}``}` `// Function to compute the SPF algorithm``static` `void` `shortestPathFaster(``int` `S, ``int` `V)``{``    ``// Create array d to store shortest distance``    ``int` `[]d = ``new` `int``[V + ``1``];` `    ``// Boolean array to check if vertex``    ``// is present in queue or not``    ``boolean` `[]inQueue = ``new` `boolean``[V + ``1``];` `    ``// Initialize the distance from source to``    ``// other vertex as Integer.MAX_VALUE(infinite)``    ``for` `(``int` `i = ``0``; i <= V; i++)``    ``{``        ``d[i] = Integer.MAX_VALUE;``    ``}``    ``d[S] = ``0``;` `    ``Queue q = ``new` `LinkedList<>();``    ``q.add(S);``    ``inQueue[S] = ``true``;` `    ``while` `(!q.isEmpty())``    ``{` `        ``// Take the front vertex from Queue``        ``int` `u = q.peek();``        ``q.remove();``        ``inQueue[u] = ``false``;` `        ``// Relaxing all the adjacent edges of``        ``// vertex taken from the Queue``        ``for` `(``int` `i = ``0``; i < graph[u].size(); i++)``        ``{` `            ``int` `v = graph[u].get(i).first;``            ``int` `weight = graph[u].get(i).second;` `            ``if` `(d[v] > d[u] + weight)``            ``{``                ``d[v] = d[u] + weight;` `                ``// Check if vertex v is in Queue or not``                ``// if not then push it into the Queue``                ``if` `(!inQueue[v])``                ``{``                    ``q.add(v);``                    ``inQueue[v] = ``true``;``                ``}``            ``}``        ``}``    ``}` `    ``// Print the result``    ``print_distance(d, V);``}` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``int` `V = ``5``;``    ``int` `S = ``1``;``    ``for` `(``int` `i = ``0``; i < graph.length; i++)``    ``{``        ``graph[i] = ``new` `Vector();``    ``}``    ` `    ``// Connect vertex a to b with weight w``    ``// addEdge(a, b, w)``    ``addEdge(``1``, ``2``, ``1``);``    ``addEdge(``2``, ``3``, ``7``);``    ``addEdge(``2``, ``4``, -``2``);``    ``addEdge(``1``, ``3``, ``8``);``    ``addEdge(``1``, ``4``, ``9``);``    ``addEdge(``3``, ``4``, ``3``);``    ``addEdge(``2``, ``5``, ``3``);``    ``addEdge(``4``, ``5``, -``3``);` `    ``// Calling shortestPathFaster function``    ``shortestPathFaster(S, V);``}``}` `// This code is contributed by 29AjayKumar`

## C#

 `// C# implementation of SPFA``using` `System;``using` `System.Collections.Generic;` `class` `GFG``{``    ``class` `pair``    ``{``        ``public` `int` `first, second;``        ``public` `pair(``int` `first, ``int` `second)``        ``{``            ``this``.first = first;``            ``this``.second = second;``        ``}``    ``}``     ` `// Graph is stored as vector of vector of pairs``// first element of pair store vertex``// second element of pair store weight``static` `List []graph = ``new` `List;`` ` `// Function to add edges in the graph``// connecting a pair of vertex(frm) and weight``// to another vertex(to) in graph``static` `void` `addEdge(``int` `frm, ``int` `to, ``int` `weight)``{`` ` `    ``graph[frm].Add(``new` `pair( to, weight ));``}`` ` `// Function to print shortest distance from source``static` `void` `print_distance(``int` `[]d, ``int` `V)``{``    ``Console.Write(``"Vertex \t\t Distance"``        ``+ ``" from source"` `+``"\n"``);`` ` `    ``for` `(``int` `i = 1; i <= V; i++)``    ``{``        ``Console.Write(``"{0} \t\t {1}\n"``, i, d[i]);``    ``}``}`` ` `// Function to compute the SPF algorithm``static` `void` `shortestPathFaster(``int` `S, ``int` `V)``{``    ``// Create array d to store shortest distance``    ``int` `[]d = ``new` `int``[V + 1];`` ` `    ``// Boolean array to check if vertex``    ``// is present in queue or not``    ``bool` `[]inQueue = ``new` `bool``[V + 1];`` ` `    ``// Initialize the distance from source to``    ``// other vertex as int.MaxValue(infinite)``    ``for` `(``int` `i = 0; i <= V; i++)``    ``{``        ``d[i] = ``int``.MaxValue;``    ``}``    ``d[S] = 0;`` ` `    ``Queue<``int``> q = ``new` `Queue<``int``>();``    ``q.Enqueue(S);``    ``inQueue[S] = ``true``;`` ` `    ``while` `(q.Count!=0)``    ``{`` ` `        ``// Take the front vertex from Queue``        ``int` `u = q.Peek();``        ``q.Dequeue();``        ``inQueue[u] = ``false``;`` ` `        ``// Relaxing all the adjacent edges of``        ``// vertex taken from the Queue``        ``for` `(``int` `i = 0; i < graph[u].Count; i++)``        ``{`` ` `            ``int` `v = graph[u][i].first;``            ``int` `weight = graph[u][i].second;`` ` `            ``if` `(d[v] > d[u] + weight)``            ``{``                ``d[v] = d[u] + weight;`` ` `                ``// Check if vertex v is in Queue or not``                ``// if not then push it into the Queue``                ``if` `(!inQueue[v])``                ``{``                    ``q.Enqueue(v);``                    ``inQueue[v] = ``true``;``                ``}``            ``}``        ``}``    ``}`` ` `    ``// Print the result``    ``print_distance(d, V);``}`` ` `// Driver code``public` `static` `void` `Main(String[] args)``{``    ``int` `V = 5;``    ``int` `S = 1;``    ``for` `(``int` `i = 0; i < graph.Length; i++)``    ``{``        ``graph[i] = ``new` `List();``    ``}``     ` `    ``// Connect vertex a to b with weight w``    ``// addEdge(a, b, w)``    ``addEdge(1, 2, 1);``    ``addEdge(2, 3, 7);``    ``addEdge(2, 4, -2);``    ``addEdge(1, 3, 8);``    ``addEdge(1, 4, 9);``    ``addEdge(3, 4, 3);``    ``addEdge(2, 5, 3);``    ``addEdge(4, 5, -3);`` ` `    ``// Calling shortestPathFaster function``    ``shortestPathFaster(S, V);``}``}` `// This code is contributed by PrinciRaj1992`

## Python3

 `# Python3 implementation of SPFA``from` `collections ``import` `deque`` ` `# Graph is stored as vector of vector of pairs``# first element of pair store vertex``# second element of pair store weight``graph ``=` `[[] ``for` `_ ``in` `range``(``100000``)]` `# Function to add edges in the graph``# connecting a pair of vertex(frm) and weight``# to another vertex(to) in graph``def` `addEdge(frm, to, weight):` `    ``graph[frm].append([to, weight])` `# Function to prshortest distance from source``def` `print_distance(d, V):``    ``print``(``"Vertex"``,``"\t"``,``"Distance from source"``)` `    ``for` `i ``in` `range``(``1``, V ``+` `1``):``        ``print``(i,``"\t"``,d[i])` `# Function to compute the SPF algorithm``def` `shortestPathFaster(S, V):` `    ``# Create array d to store shortest distance``    ``d ``=` `[``10``*``*``9``]``*``(V ``+` `1``)` `    ``# Boolean array to check if vertex``    ``# is present in queue or not``    ``inQueue ``=` `[``False``]``*``(V ``+` `1``)` `    ``d[S] ``=` `0` `    ``q ``=` `deque()``    ``q.append(S)``    ``inQueue[S] ``=` `True` `    ``while` `(``len``(q) > ``0``):` `        ``# Take the front vertex from Queue``        ``u ``=` `q.popleft()``        ``inQueue[u] ``=` `False` `        ``# Relaxing all the adjacent edges of``        ``# vertex taken from the Queue``        ``for` `i ``in` `range``(``len``(graph[u])):` `            ``v ``=` `graph[u][i][``0``]``            ``weight ``=` `graph[u][i][``1``]` `            ``if` `(d[v] > d[u] ``+` `weight):``                ``d[v] ``=` `d[u] ``+` `weight` `                ``# Check if vertex v is in Queue or not``                ``# if not then append it into the Queue``                ``if` `(inQueue[v] ``=``=` `False``):``                    ``q.append(v)``                    ``inQueue[v] ``=` `True` `    ``# Print the result``    ``print_distance(d, V)` `# Driver code``if` `__name__ ``=``=` `'__main__'``:``    ``V ``=` `5``    ``S ``=` `1` `    ``# Connect vertex a to b with weight w``    ``# addEdge(a, b, w)` `    ``addEdge(``1``, ``2``, ``1``)``    ``addEdge(``2``, ``3``, ``7``)``    ``addEdge(``2``, ``4``, ``-``2``)``    ``addEdge(``1``, ``3``, ``8``)``    ``addEdge(``1``, ``4``, ``9``)``    ``addEdge(``3``, ``4``, ``3``)``    ``addEdge(``2``, ``5``, ``3``)``    ``addEdge(``4``, ``5``, ``-``3``)` `    ``# Calling shortestPathFaster function``    ``shortestPathFaster(S, V)` `# This code is contributed by mohit kumar 29`

## Javascript

 ``
Output:
```Vertex    Distance from source
1          0
2          1
3          8
4          -1
5          -4```

Time Complexity:
Average Time Complexity: O(|E|)
Worstcase Time Complexity: O(|V|.|E|)
Note: Bound on average runtime has not been proved yet.
References: Shortest Path Faster Algorithm

My Personal Notes arrow_drop_up