124.MaxPathSum.cs

// 124. Binary Tree Maximum Path Sum
// A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them.
// A node can only appear in the sequence at most once. Note that the path does not need to pass through the root.
// The path sum of a path is the sum of the node's values in the path.
// Given the root of a binary tree, return the maximum path sum of any non-empty path.
// Example 1:
// Input: root = [1,2,3]
// Output: 6
// Explanation: The optimal path is 2 -> 1 -> 3 with a path sum of 2 + 1 + 3 = 6.
// Example 2:
// Input: root = [-10,9,20,null,null,15,7]
// Output: 42
// Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    int max = Int32.MinValue;
    public int MaxPathSum(TreeNode root) {
        RootSum(root);
        return max;
    }
    public int RootSum(TreeNode root){
        if(root == null)
            return 0;

        int lSum = 0, rSum = 0, pathSum = root.val;

        lSum = RootSum(root.left);
        pathSum = lSum > 0 ? pathSum + lSum : pathSum;

        rSum = RootSum(root.right);
        pathSum = rSum > 0 ? pathSum + rSum : pathSum;

        max = Math.Max(max, pathSum);

        return root.val  + (Math.Max(lSum,rSum) > 0 ? Math.Max(lSum,rSum) : 0) ;
    }
}