236. Lowest Common Ancestor of a Binary Tree
和235题类似,236的答案可以放在235里AC,但235是二叉搜索树,可以利用该特性优化代码实现。

/**
 * 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(NULL == left)
            return right;
        else if(NULL == right)
            return left;
        else
            return root;
        //one line judge
        //return left == NULL ? right:right == NULL ? left:root;
    }
};

运行结果:

Runtime: 16 ms, faster than 94.88% of C++ online submissions for Lowest Common Ancestor of a Binary Tree.
Memory Usage: 14.8 MB, less than 5.21% of C++ online submissions for Lowest Common Ancestor of a Binary Tree.