0124-binary-tree-maximum-path-sum.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 {
    
    class Pair {
        int maxPathSum = 0;
        int sum = 0;
        
        Pair (int sum, int maxPathSum) {
            this.sum = sum;
            this.maxPathSum = maxPathSum;
        }
    }
    
    public Pair BestPathSum(TreeNode root) {
        if (root == null) {
            return new Pair(0, Integer.MIN_VALUE);
        }
        
        Pair LeftPair = BestPathSum(root.left);
        Pair RightPair = BestPathSum(root.right);
        
        int bestPathSumFromLeft = Math.max(LeftPair.sum, 0);
        int bestPathSumFromRight = Math.max(RightPair.sum, 0);
        int currMaxPathSum = bestPathSumFromLeft + root.val + bestPathSumFromRight;
        
        int overallMaxPathSum = Math.max(currMaxPathSum, Math.max(LeftPair.maxPathSum, RightPair.maxPathSum));
        
        return new Pair(Math.max(bestPathSumFromLeft + root.val, bestPathSumFromRight + root.val), overallMaxPathSum);
    }
    
    public int maxPathSum(TreeNode root) {
        return BestPathSum(root).maxPathSum;
    }
}