Balanced Binary Tree

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

答案

这个答案用了一个小技巧
recursion返回值大于0时是在计算树的高度
recursion返回值小于0时，证明发现了2个高度差大于1的subtrees

class Solution {
    public boolean isBalanced(TreeNode root) {
        return recur(root) != -1;
    }
    
    private int recur(TreeNode r) {
        if(r == null) return 0;
        int hl = recur(r.left);
        int hr = recur(r.right);
        if(hl == -1 || hr == -1) return -1;
        if(Math.abs(hr - hl) > 1) return -1;
        return Math.max(hl, hr) + 1;
    }
}