Created Lowest_Common_Ancestor.cpp

Trees and Graphs/Lowest_Common_Ancestor.cpp

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+// C++ Program for Lowest Common Ancestor in a Binary Tree 
+// A O(n) solution to find LCA of two given values n1 and n2 
+#include <iostream> 
+#include <vector> 
+
+using namespace std; 
+
+// A Binary Tree node 
+struct Node 
+{ 
+    int key; 
+    struct Node *left, *right; 
+}; 
+
+// Utility function creates a new binary tree node with given key 
+Node * newNode(int k) 
+{ 
+    Node *temp = new Node; 
+    temp->key = k; 
+    temp->left = temp->right = NULL; 
+    return temp; 
+} 
+
+// Finds the path from root node to given root of the tree, Stores the 
+// path in a vector path[], returns true if path exists otherwise false 
+bool findPath(Node *root, vector<int> &path, int k) 
+{ 
+    // base case 
+    if (root == NULL) return false; 
+
+    // Store this node in path vector. The node will be removed if 
+    // not in path from root to k 
+    path.push_back(root->key); 
+
+    // See if the k is same as root's key 
+    if (root->key == k) 
+        return true; 
+
+    // Check if k is found in left or right sub-tree 
+    if ( (root->left && findPath(root->left, path, k)) || 
+         (root->right && findPath(root->right, path, k)) ) 
+        return true; 
+
+    // If not present in subtree rooted with root, remove root from 
+    // path[] and return false 
+    path.pop_back(); 
+    return false; 
+} 
+
+// Returns LCA if node n1, n2 are present in the given binary tree, 
+// otherwise return -1 
+int findLCA(Node *root, int n1, int n2) 
+{ 
+    // to store paths to n1 and n2 from the root 
+    vector<int> path1, path2; 
+
+    // Find paths from root to n1 and root to n1. If either n1 or n2 
+    // is not present, return -1 
+    if ( !findPath(root, path1, n1) || !findPath(root, path2, n2)) 
+          return -1; 
+
+    /* Compare the paths to get the first different value */
+    int i; 
+    for (i = 0; i < path1.size() && i < path2.size() ; i++) 
+        if (path1[i] != path2[i]) 
+            break; 
+    return path1[i-1]; 
+} 
+
+// Driver program to test above functions 
+int main() 
+{ 
+    // Let us create the Binary Tree shown in above diagram. 
+    Node * root = newNode(1); 
+    root->left = newNode(2); 
+    root->right = newNode(3); 
+    root->left->left = newNode(4); 
+    root->left->right = newNode(5); 
+    root->right->left = newNode(6); 
+    root->right->right = newNode(7); 
+    cout << "LCA(4, 5) = " << findLCA(root, 4, 5); 
+    cout << "nLCA(4, 6) = " << findLCA(root, 4, 6); 
+    cout << "nLCA(3, 4) = " << findLCA(root, 3, 4); 
+    cout << "nLCA(2, 4) = " << findLCA(root, 2, 4); 
+    return 0; 
+}