Print the path common to the two paths from the root to the two given nodes


Given a binary tree with distinct nodes(no two nodes have the same have data values). The problem is to print the path common to the two paths from the root to the two given nodes n1 and n2. If either of the nodes are not present then print “No Common Path”.


Input :          1
               /   \
              2     3
             / \   /  \
            4   5  6   7
               /    \   
              8      9

          n1 = 4, n2 = 8

Output : 1->2
Path form root to n1:

Path form root to n2:

Common Path:

Approach:The following steps are:

  1. Find the LCA(Lowest Common Ancestor) of the two nodes n1 and n2. Refer this.
  2. If LCA exits then print the path from the root to LCA. Refer this. Else print “No Common Path”.
// C++ implementation to print the path common to the 
// two paths from the root to the two given nodes
#include <bits/stdc++.h>

using namespace std;

// structure of a node of binary tree
struct Node
    int data;
    Node *left, *right;

/* Helper function that allocates a new node with the
   given data and NULL left and right pointers. */
struct Node* getNode(int data)
    struct Node *newNode = (struct Node*)malloc(sizeof(struct Node));
    newNode->data = data;
    newNode->left = newNode->right = NULL;
    return newNode;

// This function returns pointer to LCA of two given values n1 and n2.
// v1 is set as true by this function if n1 is found
// v2 is set as true by this function if n2 is found
struct Node *findLCAUtil(struct Node* root, int n1, int n2, bool &v1, bool &v2)
    // Base case
    if (root == NULL) return NULL;
    // If either n1 or n2 matches with root's data, report the presence
    // by setting v1 or v2 as true and return root (Note that if a key
    // is ancestor of other, then the ancestor key becomes LCA)
    if (root->data == n1)
        v1 = true;
        return root;
    if (root->data == n2)
        v2 = true;
        return root;
    // Look for nodes in left and right subtrees
    Node *left_lca  = findLCAUtil(root->left, n1, n2, v1, v2);
    Node *right_lca = findLCAUtil(root->right, n1, n2, v1, v2);
    // If both of the above calls return Non-NULL, then one node
    // is present in one subtree and other is present in other,
    // So this current node is the LCA
    if (left_lca && right_lca)  return root;
    // Otherwise check if left subtree or right subtree is LCA
    return (left_lca != NULL)? left_lca: right_lca;

// Returns true if key k is present in tree rooted with root
bool find(Node *root, int k)
    // Base Case
    if (root == NULL)
        return false;
    // If key k is present at root, or in left subtree 
    // or right subtree, return true
    if (root->data == k || find(root->left, k) ||  find(root->right, k))
        return true;
    // Else return false
    return false;

// This function returns LCA of n1 and n2 only if both n1 and n2 
// are present in tree, otherwise returns NULL
Node *findLCA(Node *root, int n1, int n2)
    // Initialize n1 and n2 as not visited
    bool v1 = false, v2 = false;
    // Find lca of n1 and n2
    Node *lca = findLCAUtil(root, n1, n2, v1, v2);
    // Return LCA only if both n1 and n2 are present in tree
    if (v1 && v2 || v1 && find(lca, n2) || v2 && find(lca, n1))
        return lca;
    // Else return NULL
    return NULL;

// function returns true if 
// there is a path from root to 
// the given node. It also populates 
// 'arr' with the given path
bool hasPath(Node *root, vector<int>& arr, int x)
    // if root is NULL
    // there is no path
    if (!root)
        return false;
    // push the node's value in 'arr'
    // if it is the required node
    // return true
    if (root->data == x)    
        return true;
    // else check whether there    the required node lies in the
    // left subtree or right subtree of the current node
    if (hasPath(root->left, arr, x) ||
        hasPath(root->right, arr, x))
        return true;
    // required node does not lie either in the 
    // left or right subtree of the current node
    // Thus, remove current node's value from 'arr'
    // and then return false;    
    return false;            

// function to print the path common
// to the two paths from the root 
// to the two given nodes if the nodes 
// lie in the binary tree
void printCommonPath(Node *root, int n1, int n2)
    // vector to store the common path
    vector<int> arr;
    // LCA of node n1 and n2
    Node *lca = findLCA(root, n1, n2);
    // if LCA of both n1 and n2 exists
    if (lca)
        // then print the path from root to
        // LCA node
        if (hasPath(root, arr, lca->data))
            for (int i=0; i<arr.size()-1; i++)    
                cout << arr[i] << "->";
            cout << arr[arr.size() - 1];    
    // LCA is not present in the binary tree 
    // either n1 or n2 or both are not present
        cout << "No Common Path";

// Driver program to test above
int main()
    // binary tree formation
    struct Node *root = getNode(1);
    root->left = getNode(2);
    root->right = getNode(3);
    root->left->left = getNode(4);
    root->left->right = getNode(5);
    root->right->left = getNode(6);
    root->right->right = getNode(7);
    root->left->right->left = getNode(8);
    root->right->left->right = getNode(9);
    int n1 = 4, n2 = 8;
    printCommonPath(root, n1, n2);
    return 0;



Time complexity: O(n), where n is the number of nodes in the binary tree.

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