Leetcode ——Lowest Common Ancestor of a Binary Tree

Question

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 = [3,5,1,6,2,8,null,7,4]

_______3______
   /              ___5__          ___1__

/ ? / 6 _2 0 8
/ 7 4
Example 1:

Input: root = [3,4],p = 5,q = 1
Output: 3
Explanation: The LCA of of nodes 5 and 1 is 3.
Example 2:

Input: root = [3,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.

Solution

递归。当前根节点root，其左右子树分别包含要找的节点，那么当前root，就是他们的公共祖先。

Code

/**
 * 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 || root == p || root == q) return root;
        TreeNode* left = lowestCommonAncestor(root->left,p,q);
        TreeNode* right = lowestCommonAncestor(root->right,q);
        return left == NULL ? right : right == NULL ? left : root; 
    }
};