简单题，以前居然没有贴上来

package Level1;
import Utility.TreeNode;

/**
 * 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.
 * 
 */
public class S110 {

	public static void main(String[] args) {

	}
	
	public boolean isBalanced(TreeNode root) {
        if(root == null){
        	return true;
        }
        // 如果子树高度差大于1，则不平衡
        if(Math.abs(depth(root.left)-depth(root.right)) > 1){
        	return false;
        }
        // 递归检查左子树和右子树的平衡性
        return isBalanced(root.left) && isBalanced(root.right);
    }

	// 帮助方法，返回树的高度
	private int depth(TreeNode root){
		if(root == null){
			return 0;
		}
		return 1 + Math.max(depth(root.left), depth(root.right));
	}
}

Again:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isBalanced(TreeNode root) {
        if(root == null){
            return true;
        }
        if(root.left==null && root.right==null){
            return true;
        }
        if(Math.abs(depth(root.left)-depth(root.right)) > 1){
            return false;
        }
        return isBalanced(root.left) && isBalanced(root.right);
    }
    
    public int depth(TreeNode root){
        if(root==null){
            return 0;
        }
        return 1+Math.max(depth(root.left), depth(root.right));
    }
}