Leetcode|二叉树公共祖先|235. 二叉树+BST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) {
        /** 
        函数非空返回值有两个含义：
        (1)p和q的最近公共祖先;
        (2)p或q单个节点的祖先
        **/
        if (!root) return nullptr;
        if (root == p || root == q) return root;

        TreeNode* left = lowestCommonAncestor(root->left, p, q);
        TreeNode* right = lowestCommonAncestor(root->right, p, q);
        // 1.左右子树无p和q节点,则返回nullptr——含义(1,2)
        if (!left && !right) return nullptr;
        // 2.左右子树中分别有p和q节点,则返回root——含义(2)
        if (left && right) return root;
        // 3.仅有1个子树(如左子树)有单个节点祖先(返回left/root) ∩ 两节点公共祖先(返回left) = 返回left——含义(1,2)
        return left ? left : right;
    }
};

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (!root) return nullptr;
        if (root == p || root == q) return root;
        TreeNode* left = nullptr;
        TreeNode* right = nullptr;
        if (root->val > p->val || root->val > q->val)
            left = lowestCommonAncestor(root->left, p, q);
        if (root->val < p->val || root->val < q->val)
            right = lowestCommonAncestor(root->right, p, q);
        if (left && right) return root;
        if (!left && !right) return nullptr;
        return left ? left : right;
    }
};