add: MaximumPathSum

Author: Jer3myYu

java/Trees/MaximumPathSum.java

@@ -0,0 +1,39 @@
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class MaximumPathSum {
+    int maxSum = Integer.MIN_VALUE;
+
+    public int maxPathSum(TreeNode root) {
+        maxPathSumHelper(root);
+        return maxSum;
+    }
+
+    private int maxPathSumHelper(TreeNode node) {
+        // Base case: null nodes have no path sum.
+        if (node == null) {
+            return 0;
+        }
+        // Collect the maximum gain we can attain from the left and right 
+        // subtrees, setting them to 0 if they're negative.
+        int leftSum = Math.max(maxPathSumHelper(node.left), 0);
+        int rightSum = Math.max(maxPathSumHelper(node.right), 0);
+        // Update the overall maximum path sum if the current path sum is 
+        // larger.
+        maxSum = Math.max(maxSum, node.val + leftSum + rightSum);
+        // Return the maximum sum of a single, continuous path with the 
+        // current node as an endpoint.
+        return node.val + Math.max(leftSum, rightSum);
+    }
+}