[LeetCode] Balanced Binary Tree

Thought:
To prevent redundant computation, we'd better first examine the flag of the left subtree and then the right one.
-1 means unbalanced and if not -1 it means the height.

Code:
public class Solution {
    public boolean isBalanced(TreeNode root) {
        if(root == null) {
            return true;
        }else{
            return flag(root) == -1? false: true; 
        }       
    }
    public int flag(TreeNode root) {
        if(root == null) {
            return 0;
        }else{           
            int lFlag = flag(root.left);
            if(lFlag == -1) return -1;
            int rFlag = flag(root.right);
            if(rFlag == -1) return -1;
            return Math.abs(lFlag - rFlag) <= 1? Math.max(lFlag, rFlag) + 1: -1;       
        }
    }
}