Balanced Binary Tree 判断二叉树是否是高度相差不超过1的高度平衡的二叉树

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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 ofevery node never differ by more than 1.

转载出处:http://blog.csdn.net/ljiabin/article/details/40021481

递归的用发，我真是笨啊

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

下边这种写法，把计算出来的值保存在了树节点里边。利用了TreeNode结构中的val，用它来记录以当前结点为根的子树的高度，避免多次计算。

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

下边写法的出处见这儿：http://www.programcreek.com/2013/02/leetcode-balanced-binary-tree-java/

写成这样是对的

public class Solution {
    public boolean isBalanced(TreeNode root) {
        if(root==null) return true;
        if (depth(root) == -1)    return false;
        return true;
    }
    private int depth(TreeNode root){
        if(root == null) return 0;
        int left=depth(root.left); //没有这两句，下边的竟然超时了
        int right=depth(root.right);
        if(left == -1 || right == -1)   return -1;
        if(Math.abs(left-right)>1) return -1;
        return Math.max(left,right)+1;
    }
}

但是写成这样却是超时的

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

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