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.

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;
    }
};