LeetCode Java Solutions

124. Binary Tree Maximum Path Sum

Hard

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int maxPathSum = Integer.MIN_VALUE;    
    public int maxPathSum(TreeNode root) {
        pathSum(root); 
        return maxPathSum;
    }
    private int pathSum(TreeNode node){
        if(node == null){
            return 0;
        }
        int left = Math.max(0, pathSum(node.left));
        int right = Math.max(0, pathSum(node.right));
        maxPathSum = Math.max(maxPathSum, left+right+node.val);
        return Math.max(left,right)+node.val;
    }
}