Create BinaryTreeMaximumPathSum.java

124

Author: leo414

BinaryTreeMaximumPathSum.java

@@ -0,0 +1,53 @@
+package leetcode;
+
+/**
+ * Project Name : Leetcode
+ * Package Name : leetcode
+ * File Name : BinaryTreeMaximumPathSum
+ * Creator : Leo
+ * Description : 124. Binary Tree Maximum Path Sum
+ */
+public class BinaryTreeMaximumPathSum {
+    /**
+     * Given a binary tree, find the maximum path sum.
+
+     For this problem, a path is defined as any sequence of nodes from some starting node to any node
+     in the tree along the parent-child connections. The path must contain at least one node and does not
+     need to go through the root.
+
+     For example:
+     Given the below binary tree,
+
+       1
+      / \
+     2   3
+     Return 6.
+
+          3
+         / \
+        9  20
+      /  \
+     15   7
+
+
+     time : O(n)
+     space : O(n)
+
+     */
+
+    int res;
+
+    public int maxPathSum(TreeNode root) {
+        if (root == null) return 0;
+        res = Integer.MIN_VALUE;
+        helper(root);
+        return res;
+    }
+    public int helper(TreeNode root) {
+        if (root == null) return 0;
+        int left = Math.max(0, helper(root.left));
+        int right = Math.max(0, helper(root.right));
+        res = Math.max(res, left + right + root.val);
+        return Math.max(left, right) + root.val;
+    }
+}