平衡树：对于每一个节点它的左右子树深度差不能超过1
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isBalanced(TreeNode root) {
if(root == null) return true;
return (Math.abs(getDepth(root.left)-getDepth(root.right))<=1)
&& isBalanced(root.left) && isBalanced(root.right);//递归判断每个节点的子数深度
}
public int getDepth(TreeNode root){//求深度
if(root == null) return 0;
return Math.max(getDepth(root.left),getDepth(root.right))+1;
}
}