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.

思路一：

当一个二叉树的左子树和右子树都是平衡二叉树，且左右子树深度之差小于等于1，即可判断该二叉树为平衡二叉树。

public boolean isBalanced(TreeNode root) {
        if(root == null) {
            return true;
        }
        boolean cut = false;
        if(root.right == null || root.left == null) {
            cut = true;
        }
        return isBalanced(root.left) && isBalanced(root.right)
                && Math.abs(getDepth(root.left, cut) - getDepth(root.right, cut)) <= 1;
    }
    public int getDepth(TreeNode root, boolean cut) {
        if(root == null) {
            return 0;
        }
        if(cut && (root.left != null || root.right != null)) {
            //当前子树至少是2，另一颗子树是0的情况，直接返回
            return 2;
        }
        return Math.max(getDepth(root.left, false), getDepth(root.right, false)) + 1;
    }