需要一个辅助方法，计算树的高度

public int height(TreeNode root) {
        if(root == null) return 0;
        int leftHeight = height(root.left);
        int rightHeight = height(root.right);
        return leftHeight>rightHeight?(leftHeight+1):(rightHeight+1);
    }

判断树是否为平衡二叉树

public class Solution {
    public boolean isBalanced(TreeNode root) {
        if(root == null) return true;
        int diff = height(root.left) - height(root.right);
        if(diff<-1 || diff>1) return false;
        return isBalanced(root.left)&&isBalanced(root.right);
    }
    
}