算法作業_24(2017.5.23第十四周)

563. Binary Tree Tilt

Given a binary tree, return the tilt of the whole tree.

The tilt of a tree node is defined as the absolute difference between the sum of all left subtree node values and the sum of all right subtree node values. Null node has tilt 0.

The tilt of the whole tree is defined as the sum of all nodes' tilt.

Example:

Input: 
         1
       /   \
      2     3
Output: 1
Explanation: 
Tilt of node 2 : 0
Tilt of node 3 : 0
Tilt of node 1 : |2-3| = 1
Tilt of binary tree : 0 + 0 + 1 = 1

剛開始總是理解錯題意，導致總是錯誤。這道題其實是要求 求出各個節點左右子樹的差的絕對值，將這些絕對值求和並返回

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private int sum = 0;  
    public int findTilt(TreeNode root) {  
        helper(root);  
        return sum;  
    }  
    private int helper(TreeNode root) {  
        if (root == null)  
            return 0;  
        int left = helper(root.left);  
        int right = helper(root.right);  
        sum += Math.abs(left - right);  
        return root.val + left + right;  
    }  
}