【仍需研究】Lintcode 1172: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.

Notice

1.The sum of node values in any subtree won't exceed the range of 32-bit integer.
2.All the tilt values won't exceed the range of 32-bit integer.

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

解题思路：

本身对递归理解就不深，所以这道题也是翻看了很多其他人的博客得出来的解，需要加强理解。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: the root
     * @return: the tilt of the whole tree
     */
     int sum = 0;
    int findTilt(TreeNode * root) 
    {
        // Write your code here
        helper(root);
        return sum;
    }
    
    int helper(TreeNode * root)
    {
        if(root == NULL)
            return 0;
            
        int left = helper(root->left);
        int right = helper(root->right);
        sum += abs(left - right);
        
        return left + right + root->val;
    }
};