# Shortest path to reach one prime to other by changing single digit at a time

• Difficulty Level : Hard
• Last Updated : 16 Sep, 2019

Given two four digit prime numbers, suppose 1033 and 8179, we need to find the shortest path from 1033 to 8179 by altering only single digit at a time such that every number that we get after changing a digit is prime. For example a solution is 1033, 1733, 3733, 3739, 3779, 8779, 8179

Examples:

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```Input : 1033 8179
Output :6

Input : 1373 8017
Output : 7

Input  :  1033 1033
Output : 0
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

The question can be solved by BFS and it is a pretty interesting to solve as a starting problem for beginners. We first find out all 4 digit prime numbers till 9999 using technique of Sieve of Eratosthenes. And then using those numbers formed the graph using adjacency list. After forming the adjacency list, we used simple BFS to solve the problem.

## C++

 `// CPP program to reach a prime number from ``// another by changing single digits and ``// using only prime numbers.``#include `` ` `using` `namespace` `std;`` ` `class` `graph {``    ``int` `V; ``    ``list<``int``>* l; ``public``:``    ``graph(``int` `V)``    ``{``        ``this``->V = V;``        ``l = ``new` `list<``int``>[V];``    ``}``    ``void` `addedge(``int` `V1, ``int` `V2) ``    ``{``        ``l[V1].push_back(V2);``        ``l[V2].push_back(V1);``    ``}``    ``int` `bfs(``int` `in1, ``int` `in2);``};`` ` `// Finding all 4 digit prime numbers``void` `SieveOfEratosthenes(vector<``int``>& v) ``{``    ``// Create a boolean array "prime[0..n]" and initialize``    ``// all entries it as true. A value in prime[i] will``    ``// finally be false if i is Not a prime, else true.``    ``int` `n = 9999;``    ``bool` `prime[n + 1];``    ``memset``(prime, ``true``, ``sizeof``(prime));`` ` `    ``for` `(``int` `p = 2; p * p <= n; p++) {`` ` `        ``// If prime[p] is not changed, then it is a prime``        ``if` `(prime[p] == ``true``) {`` ` `            ``// Update all multiples of p``            ``for` `(``int` `i = p * p; i <= n; i += p)``                ``prime[i] = ``false``;``        ``}``    ``}`` ` `    ``// Forming a vector of prime numbers``    ``for` `(``int` `p = 1000; p <= n; p++)``        ``if` `(prime[p])``            ``v.push_back(p); ``}`` ` `// in1 and in2 are two vertices of graph which are ``// actually indexes in pset[]``int` `graph::bfs(``int` `in1, ``int` `in2) ``{``    ``int` `visited[V];``    ``memset``(visited, 0, ``sizeof``(visited));``    ``queue<``int``> que;``    ``visited[in1] = 1;``    ``que.push(in1);``    ``list<``int``>::iterator i;``    ``while` `(!que.empty()) {``        ``int` `p = que.front();``        ``que.pop();``        ``for` `(i = l[p].begin(); i != l[p].end(); i++) {``            ``if` `(!visited[*i]) {``                ``visited[*i] = visited[p] + 1;``                ``que.push(*i);``            ``}``            ``if` `(*i == in2) {``                ``return` `visited[*i] - 1;``            ``}``        ``}``    ``}``}`` ` `// Returns true if num1 and num2 differ ``// by single digit.``bool` `compare(``int` `num1, ``int` `num2)``{``    ``// To compare the digits``    ``string s1 = to_string(num1);``    ``string s2 = to_string(num2);``    ``int` `c = 0;``    ``if` `(s1[0] != s2[0])``        ``c++;``    ``if` `(s1[1] != s2[1])``        ``c++;``    ``if` `(s1[2] != s2[2])``        ``c++;``    ``if` `(s1[3] != s2[3])``        ``c++;`` ` `    ``// If the numbers differ only by a single``    ``// digit return true else false``    ``return` `(c == 1);``}`` ` `int` `shortestPath(``int` `num1, ``int` `num2)``{``    ``// Generate all 4 digit``    ``vector<``int``> pset; ``    ``SieveOfEratosthenes(pset);`` ` ` ` `    ``// Create a graph where node numbers are indexes``    ``// in pset[] and there is an edge between two ``    ``// nodes only if they differ by single digit.``    ``graph g(pset.size()); ``    ``for` `(``int` `i = 0; i < pset.size(); i++) ``        ``for` `(``int` `j = i + 1; j < pset.size(); j++) ``            ``if` `(compare(pset[i], pset[j]))``                ``g.addedge(i, j);     ``     ` ` ` `    ``// Since graph nodes represent indexes of numbers``    ``// in pset[], we find indexes of num1 and num2.``    ``int` `in1, in2;``    ``for` `(``int` `j = 0; j < pset.size(); j++) ``        ``if` `(pset[j] == num1)``            ``in1 = j; ``    ``for` `(``int` `j = 0; j < pset.size(); j++) ``        ``if` `(pset[j] == num2)``            ``in2 = j; `` ` `    ``return` `g.bfs(in1, in2);``}`` ` `// Driver code``int` `main()``{``    ``int` `num1 = 1033, num2 = 8179;``    ``cout << shortestPath(num1, num2);``    ``return` `0;``}`

## Python3

 `# Python3 program to reach a prime number ``# from another by changing single digits  ``# and using only prime numbers.``import` `queue `` ` `class` `Graph: ``     ` `    ``def` `__init__(``self``, V):``        ``self``.V ``=` `V; ``        ``self``.l ``=` `[[] ``for` `i ``in` `range``(V)]``         ` `    ``def` `addedge(``self``, V1, V2):``        ``self``.l[V1].append(V2); ``        ``self``.l[V2].append(V1);`` ` `    ``# in1 and in2 are two vertices of graph  ``    ``# which are actually indexes in pset[] ``    ``def` `bfs(``self``, in1, in2):``        ``visited ``=` `[``0``] ``*` `self``.V``        ``que ``=` `queue.Queue()``        ``visited[in1] ``=` `1``        ``que.put(in1)``        ``while` `(``not` `que.empty()): ``            ``p ``=` `que.queue[``0``] ``            ``que.get()``            ``i ``=` `0``            ``while` `i < ``len``(``self``.l[p]):``                ``if` `(``not` `visited[``self``.l[p][i]]):``                    ``visited[``self``.l[p][i]] ``=` `visited[p] ``+` `1``                    ``que.put(``self``.l[p][i])``                ``if` `(``self``.l[p][i] ``=``=` `in2):``                    ``return` `visited[``self``.l[p][i]] ``-` `1``                ``i ``+``=` `1``     ` `    ``# Returns true if num1 and num2 ``    ``# differ by single digit.``     ` `# Finding all 4 digit prime numbers ``def` `SieveOfEratosthenes(v):``     ` `    ``# Create a boolean array "prime[0..n]" and  ``    ``# initialize all entries it as true. A value ``    ``# in prime[i] will finally be false if i is ``    ``# Not a prime, else true. ``    ``n ``=` `9999``    ``prime ``=` `[``True``] ``*` `(n ``+` `1``)`` ` `    ``p ``=` `2``    ``while` `p ``*` `p <``=` `n:`` ` `        ``# If prime[p] is not changed, ``        ``# then it is a prime ``        ``if` `(prime[p] ``=``=` `True``):`` ` `            ``# Update all multiples of p``            ``for` `i ``in` `range``(p ``*` `p, n ``+` `1``, p):``                ``prime[i] ``=` `False``        ``p ``+``=` `1`` ` `    ``# Forming a vector of prime numbers``    ``for` `p ``in` `range``(``1000``, n ``+` `1``):``        ``if` `(prime[p]): ``            ``v.append(p)``     ` `def` `compare(num1, num2):``     ` `    ``# To compare the digits ``    ``s1 ``=` `str``(num1) ``    ``s2 ``=` `str``(num2)``    ``c ``=` `0``    ``if` `(s1[``0``] !``=` `s2[``0``]):``        ``c ``+``=` `1``    ``if` `(s1[``1``] !``=` `s2[``1``]):``        ``c ``+``=` `1``    ``if` `(s1[``2``] !``=` `s2[``2``]):``        ``c ``+``=` `1``    ``if` `(s1[``3``] !``=` `s2[``3``]): ``        ``c ``+``=` `1`` ` `    ``# If the numbers differ only by a single ``    ``# digit return true else false ``    ``return` `(c ``=``=` `1``)``     ` `def` `shortestPath(num1, num2):``     ` `    ``# Generate all 4 digit ``    ``pset ``=` `[] ``    ``SieveOfEratosthenes(pset) `` ` `    ``# Create a graph where node numbers ``    ``# are indexes in pset[] and there is ``    ``# an edge between two nodes only if ``    ``# they differ by single digit. ``    ``g ``=` `Graph(``len``(pset))``    ``for` `i ``in` `range``(``len``(pset)):``        ``for` `j ``in` `range``(i ``+` `1``, ``len``(pset)):``            ``if` `(compare(pset[i], pset[j])): ``                ``g.addedge(i, j)     `` ` `    ``# Since graph nodes represent indexes ``    ``# of numbers in pset[], we find indexes``    ``# of num1 and num2. ``    ``in1, in2 ``=` `None``, ``None``    ``for` `j ``in` `range``(``len``(pset)):``        ``if` `(pset[j] ``=``=` `num1):``            ``in1 ``=` `j``    ``for` `j ``in` `range``(``len``(pset)):``        ``if` `(pset[j] ``=``=` `num2): ``            ``in2 ``=` `j`` ` `    ``return` `g.bfs(in1, in2)`` ` `# Driver code ``if` `__name__ ``=``=` `'__main__'``:`` ` `    ``num1 ``=` `1033``    ``num2 ``=` `8179``    ``print``(shortestPath(num1, num2))`` ` `# This code is contributed by PranchalK`

Output :
`6`

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