124. Binary Tree Maximum Path Sum.cpp

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




/**
 * 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 maxPathSumUtil(TreeNode* root, int &res){
        if(!root) return 0;

        int l=maxPathSumUtil(root->left, res);
        int r=maxPathSumUtil(root->right, res);

        int temp=max(max(l,r)+root->val, root->val);
        int ans=max(temp, l+r+root->val);
        res=max(res, ans);

        return temp;
    }

    int maxPathSum(TreeNode* root) {
        int res=INT_MIN;
        maxPathSumUtil(root, res);
        return res;
    }
};