LeetCode 124. Binary Tree Maximum Path Sum

#include <iostream>
#include <climits>
using namespace std;
/*
  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 does not need 
  to go through the root.
  Example:
        1
      2   3
  return 1 + 2 + 3 = 6
*/

/*
  struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  }
*/
int maxPathSum(TreeNode* root, int& maxValue) {
  if(!root) return 0;
  int leftLegValue = maxPathSum(root->left, maxValue);
  int rightLeftValue = maxPathSum(root->right, maxValue);

  int m = root->val;
  int longerLeg = max(leftLegValue, rightLegValue);

  // maximum result which contains the root.
  int result = max(m, m + longerLeg);

  int maxPath = m;
  maxPath = max(maxPath, maxPath + leftLegValue);
  maxPath = max(maxPath, maxPath + rightLegValue);
  maxValue = max(maxValue, maxPath);
  return result;
}
int maxPathSum(TreeNode* root) {
  int maxValue = INT_MIN;
  maxPathSum(root, maxValue);
  return maxValue;
}