Leet Code: Problem #124 Binary Tree Maximum Path Sum

Given a non-empty 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.

Example 1:

Input: [1,2,3]

       1
      / \
     2   3

Output: 6

Example 2:

Input: [-10,9,20,null,null,15,7]

   -10
   / \
  9  20
    /  \
   15   7

Output: 42

Solution in C++:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int maxPathSum(TreeNode* root) {
        int ret = INT_MIN;
        cal(root, ret);
        return ret;
    }    
    int cal(TreeNode* root, int &ret){
        if(!root)
            return 0;
        int left = cal(root->left, ret);
        int right = cal(root->right, ret);        
        int maxlr = max(left, right);        
        int side_max = max(maxlr + root->val, root->val);        
        int subr_tree_max = max(side_max, left + right + root->val);        
        ret = max(ret, subr_tree_max);        
        return side_max;
    }
};