236. Lowest Common Ancestor of a Binary Tree

• Difficulty: Medium

• Topics: Tree

• Similar Questions:

  • Lowest Common Ancestor of a Binary Search Tree

Problem:

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Given the following binary tree:  root = [3,5,1,6,2,0,8,null,null,7,4]

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

Note:

• All of the nodes' values will be unique.
• p and q are different and both values will exist in the binary tree.

Solutions:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (root == NULL)   return NULL;
        if (root == p)  return p;
        if (root == q)  return q;

        TreeNode* leftRet = lowestCommonAncestor(root->left, p, q);
        TreeNode* rightRet = lowestCommonAncestor(root->right, p, q);

        if (leftRet == NULL && rightRet == NULL)    return NULL;
        if (leftRet == NULL)    return rightRet;
        if (rightRet == NULL)   return leftRet;

        return root;

    }
};