236. Lowest Common Ancestor of a Binary Tree LeetCode

题意：求二叉树上两个节点p,q的最近公共祖先。
题解：dfs求解，当遍历到p或者q的时候往上返回p或者q，否则返回NULL，当遇到第一个左右子树都有返回值的那个节点时，就找到了他们的最近公共祖先。

/** * 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 || root == q) return root;        TreeNode *left = lowestCommonAncestor(root->left,p,q);        TreeNode *right = lowestCommonAncestor(root->right,p,q);        if(left && right) return root;        else if(left) return left;        else return right;    }};