Leetcode | 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.

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。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode *root) {
        return getLayer(root, 1) != -1;
    }
    
    int getLayer(TreeNode *root, int layer) {
        if (root == NULL) return layer;
        int l = getLayer(root->left, layer + 1);
        if (l == -1) return -1;
        int r = getLayer(root->right, layer + 1);
        if (r == -1) return -1;
        if (abs(l - r) > 1) return -1;
        return l > r ? l : r;
    }
};

 3-rd try:

class Solution {
public:
    bool isBalanced(TreeNode *root) {
       int height = 0;
       return recurse(root, height);
    }
    
    bool recurse(TreeNode *root, int &height) {
        if (root == NULL) return true;
        int lheight = 0, rheight = 0;
        if (!recurse(root->left, lheight) || !recurse(root->right, rheight)) return false;
        height = 1 + (lheight > rheight ? lheight : rheight);
        return abs(lheight - rheight) <= 1;
    }
};