Lowest Common Ancestor in Parent Array Representation

Given a binary tree represented as parent array, find Lowest Common Ancestor between two nodes ‘m’ and ‘n’.

In the above diagram, LCA of 10 and 14 is 12 and LCA of 10 and 12 is 12.

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

(1) Make a parent array and store the parent of ith node in it. Parent of root node should be -1.
(2) Now, access all the nodes from the desired node ‘m’ till root node and mark them visited.
(3) Lastly, access all the nodes from the desired node ‘n’ till first visited node comes.
(4) This node is the lowest common ancestor

C++

// CPP program to find LCA in a tree represented 
// as parent array. 
#include <bits/stdc++.h> 
using namespace std; 
  
// Maximum value in a node 
const int MAX = 1000; 
  
// Function to find the Lowest common ancestor 
int findLCA(int n1, int n2, int parent[]) 
{ 
    // Create a visited vector and mark 
    // all nodes as not visited. 
    vector<bool> visited(MAX, false); 
  
    visited[n1] = true; 
  
    // Moving from n1 node till root and 
    // mark every accessed node as visited 
    while (parent[n1] != -1) { 
        visited[n1] = true; 
  
        // Move to the parent of node n1 
        n1 = parent[n1]; 
    } 
  
    visited[n1] = true; 
  
    // For second node finding the first 
    // node common 
    while (!visited[n2]) 
        n2 = parent[n2]; 
  
    return n2; 
} 
  
// Insert function for Binary tree 
void insertAdj(int parent[], int i, int j) 
{ 
    parent[i] = j; 
} 
  
// Driver Functiom 
int main() 
{ 
    // Maximum capacity of binary tree 
    int parent[MAX]; 
  
    // Root marked 
    parent[20] = -1; 
    insertAdj(parent, 8, 20); 
    insertAdj(parent, 22, 20); 
    insertAdj(parent, 4, 8); 
    insertAdj(parent, 12, 8); 
    insertAdj(parent, 10, 12); 
    insertAdj(parent, 14, 12); 
  
    cout << findLCA(10, 14, parent); 
  
    return 0; 
} 

Python3

# Python 3 program to find LCA in a
# tree represented as parent array.

# Maximum value in a node
MAX = 1000

# Function to find the Lowest
# common ancestor
def findLCA(n1, n2, parent):

# Create a visited vector and mark
# all nodes as not visited.
visited = [False for i in range(MAX)]

visited[n1] = True

# Moving from n1 node till root and
# mark every accessed node as visited
while (parent[n1] != -1):
visited[n1] = True

# Move to the parent of node n1
n1 = parent[n1]

visited[n1] = True

# For second node finding the
# first node common
while (visited[n2] == False):
n2 = parent[n2]

return n2

# Insert function for Binary tree
def insertAdj(parent, i, j):
parent[i] = j

# Driver Code
if __name__ ==’__main__’:

# Maximum capacity of binary tree
parent = [0 for i in range(MAX)]

# Root marked
parent[20] = -1
insertAdj(parent, 8, 20)
insertAdj(parent, 22, 20)
insertAdj(parent, 4, 8)
insertAdj(parent, 12, 8)
insertAdj(parent, 10, 12)
insertAdj(parent, 14, 12)

print(findLCA(10, 14, parent))

# This code is contributed by
# Surendra_Gangwar

Output:

Time Complexity – The time complexity of the above algorithm is O(log n) as it requires O(log n) time in searching.

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