[124 Binary Tree Maximum Path Sum] #leetcode

124_0803.java

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