LeetCode-235&236. 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 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 = [3,5,1,6,2,0,8,null,null,7,4]

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1 Output: 3 Explanation: The LCA of nodes ​​5​​​ and ​​1​​​ is ​​3.​​

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, 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.

题解：

先判断是否root为p或者q，然后递归搜索左右子树，如果左右子树各包含一个p，q那么返回root，否则的话返回包含pq的左子树或者右子树。

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 && right != NULL) {
            return root;
        }
        if (left == NULL && right == NULL) {
            return NULL;
        }
        if (left == NULL && right != NULL) {
            return right;
        }
        if (left != NULL && right == NULL) {
            return left;
        }
        return NULL;
    }
};