《从头再来》剑指offer.68II 二叉树的最近公共祖先

/**
 * 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) {
        if(root == nullptr) return nullptr;
        //少判断了当前节点就是需要的节点！！！！
        if(root == p || root == q)  return root;

        //取得左、右子树
        TreeNode* leftTree = lowestCommonAncestor(root->left, p, q);
        TreeNode* rightTree = lowestCommonAncestor(root->right, p, q);

        if(leftTree && rightTree)   return root;
        else if(leftTree && !rightTree) return leftTree;
        else if(!leftTree && rightTree) return rightTree;
        else if(!leftTree && !rightTree)    return nullptr;


        return root;//最后为什么要return root？
    }
};