/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 /*
 
 前序遍历，自底向上，时间复杂度为O(n)
 
 */
class height{
    int h;
    boolean flag;

    public height(int h, boolean flag){
        this.h = h;
        this.flag = flag;
    }
}
class Solution {
    public static height judge(TreeNode root){
        if(root == null)return new height(-1, true);
        height left = judge(root.left);
        if(!left.flag)return new height(-1,false);
        height right = judge(root.right);
        if(!right.flag)return new height(-1,false);

        if(Math.abs(left.h-right.h) > 1)return new height(-1,false);
        return new height(Math.max(left.h,right.h)+1,true);
    }
    public boolean isBalanced(TreeNode root) {
        return judge(root).flag;
    }
}