leetcode | Balanced Binary Tree

Balanced Binary Tree : https://leetcode.com/problems/balanced-binary-tree/

given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

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解析：
深度优先搜索 DFS：
本题关键点：每个节点的子树都是平衡，整棵树才是平衡的。那么当某一个子树是非平衡时，必须做出标记，以使在计算该非平衡子树的父节点深度时能直接查看，由于树的深度是非负的，因此当存在不平衡子树时，对该不平衡子树的深度标为 -1，然后每次计算深度时，查看是否有深度 -1 的子树。

/** * 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:    bool isBalanced(TreeNode* root) {        if (getDepth(root) >= 0)            return true;        else            return false;    }    int getDepth(TreeNode* root) {        if (root == NULL)            return 0;        int left_depth = getDepth(root->left);        int right_depth = getDepth(root->right);        if (left_depth < 0 || right_depth < 0 || abs(left_depth-right_depth) > 1)            return -1;  // left_depth < 0 || right_depth < 0 代表左右子树已经不是平衡树了        return max(left_depth, right_depth)+1;    }};