124-Binary-Tree-Maximum-Path-Sum.java

package java;

/**
 * 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 {
    
    int[] res = new int[1];
    
    public int maxPathSum(TreeNode root) {
        res[0] = root.val;
        dfs(root);
        return res[0];
    }
    
    private int dfs(TreeNode node) {
        if (node == null) return 0;
        
        int leftMax = dfs(node.left);
        int rightMax = dfs(node.right);
        
        leftMax = Math.max(leftMax, 0);
        rightMax = Math.max(rightMax, 0);
        
        int allMax = leftMax + rightMax + node.val;
        res[0] = Math.max(res[0], allMax);
        
        
        return node.val + Math.max(leftMax, rightMax);
    }
}