二叉树的最近公共祖先

Note: 核心想法是看左边有没有，右边有没有。如果左边没有，说明在右边；右边没有，说明在左边；两边都有说明是"root"。

/**
 * 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, p, q);
        
        if(left == NULL) {
            return right;
        }
        if(right == NULL) {
            return left;
        }
        return root;
    }
};