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.

判断左右子树的高度相差是否大于1. 

树的高度取决于左右子树的高度的最大值。

1 /** 2  * Definition for binary tree 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     bool isBalanced(TreeNode *root) {13         return getLayer(root, 1) != -1;14     }15     16     int getLayer(TreeNode *root, int layer) {17         if (root == NULL) return layer;18         int l = getLayer(root->left, layer + 1);19         if (l == -1) return -1;20         int r = getLayer(root->right, layer + 1);21         if (r == -1) return -1;22         if (abs(l - r) > 1) return -1;23         return l > r ? l : r;24     }25 };

 3-rd try:

1 class Solution { 2 public: 3     bool isBalanced(TreeNode *root) { 4        int height = 0; 5        return recurse(root, height); 6     } 7      8     bool recurse(TreeNode *root, int &height) { 9         if (root == NULL) return true;10         int lheight = 0, rheight = 0;11         if (!recurse(root->left, lheight) || !recurse(root->right, rheight)) return false;12         height = 1 + (lheight > rheight ? lheight : rheight);13         return abs(lheight - rheight) <= 1;14     }15 };