LeetCode Lowest Common Ancestor Of A Binary Tree


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 v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        /              \
     ___5__          ___1__
    /      \        /      \
    6      _2       0       8
          /  \
          7   4

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

The original problem is here.

The original code is here.

My Solution

I solve this problem in C++, as below:

*Lowest Common Ancestor of a Binary Tree
*Author: shuaijiang
*Email: zhaoshuaijiang8@gmail.com
using namespace std;

 * 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 {
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
      if (root == NULL) return NULL;
         if (root == p || root == q) return root;
         TreeNode *L = lowestCommonAncestor(root->left, p, q);
         TreeNode *R = lowestCommonAncestor(root->right, p, q);
         if (L && R) return root;
         return L ? L : R;


To solve the problem, recursion is needed.