算法1

递归

从在root树里面找 p和q的公共祖先;
递归:
p和q都不是root, 直接在子树里面递归:
否则额外考虑下p和q是root的情况。

C++ 代码

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (!root) return root;
        if (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 left == NULL ? right : left;
        }
        return root;
    } 
};