124. Binary Tree Maximum Path Sum

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private int maxValue = Integer.MIN_VALUE;
    public int maxPathSum(TreeNode root) {
        recursion(root);
        return maxValue;
    }
    
    // returns the max value from current node to left or right
    private int recursion(TreeNode node){
        if(node == null) return 0;
        int left = Math.max(0, recursion(node.left));
        int right = Math.max(0,recursion(node.right));
        // check if the path value from let to node to right is larger than maxValue
        maxValue=Math.max(maxValue, node.val + left + right);
        return node.val+Math.max(left, right);
    }
}