563. Binary Tree Tilt

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https://leetcode.com/problems/binary-tree-tilt/description/

挺好的一個題目，審題不清的話很容易做錯。主要是tilt of whole tree 的定義是sum of all node‘s tilt 而不是想當然的tilt of root.

一開是我就以為是簡單的tilt of root 導致完全錯誤。思路其實可以看作是求sum of children 的變種，只是這裡不僅要跟蹤每個子樹的sum 還要累計上他們的tilt。

 

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    int tiltIter(TreeNode* root, int *t) {        if (root == nullptr) {            return 0;        }                int leftSum = 0;        int rightSum = 0;        if (root->left != nullptr) {            leftSum = tiltIter(root->left, t);        }        if (root->right != nullptr) {            rightSum = tiltIter(root->right, t);        }        //tilt of the current node;        int tilt = abs(leftSum - rightSum);        *t += tilt;        return root->val + leftSum + rightSum;    }        int findTilt(TreeNode* root) {        int tilt = 0;        tiltIter(root, &tilt);        return tilt;    }};

 

563. Binary Tree Tilt