【LeetCode】124. Binary Tree Maximum Path Sum

Given a binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

For example:
Given the below binary tree,

       1
      / 
     2   3

Return 6.

题解：

　　感觉解题思路有点类似于动态规划，一个变量用于表示以此节点为根的最大路径和，另一个变量为全局变量，全局更新最大值

class Solution {
public:
    int maxPathSum(TreeNode* root) {
        if (!root)
            return 0;
        int res = INT_MIN;
        helper(root, res);
        return res;
    }
    
    int helper(TreeNode* root, int& res) {
        if (!root)
            return 0;
        
        int lmax = max(helper(root->left, res), 0);
        int rmax = max(helper(root->right, res), 0);
        res = max(lmax + rmax + root->val, res);
        return max(lmax, rmax) + root->val;
    }
};