leetcode110~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.

判断一棵树是否为平衡二叉树。
平衡二叉树：对于树中每一个节点来说，左右子树的高度差不能超过1.

关键：使用递归求出子树高度，然后做差比较即可。返回-1代表非平衡。

在给出本题代码之前，先附上求树高度的代码，便于理解：

public int getHeight(TreeNode root){    if(root==null) return 0;    int left = getHeight(root.left);    int right = getHeight(root.right);    return Math.max(left,right)+1;}

只需对上面代码稍微修改一下即可：

public class IsBalanced {    public boolean isBalanced(TreeNode root) {        if(getHeight(root)==-1) {            return false;        } else {            return true;        }    }    private int getHeight(TreeNode root) {        if(root==null) return 0;        int left = getHeight(root.left);        int right = getHeight(root.right);        if(left==-1 || right==-1 || Math.abs(left-right)>1) {            return -1;        }        return Math.max(left, right)+1;    }}