Create 1120-maximum-average-subtree.js

Author: everthis

1120-maximum-average-subtree.js

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+/**
+
+Given the root of a binary tree, find the maximum average value of any subtree of that tree.
+(A subtree of a tree is any node of that tree plus all its descendants.
+The average value of a tree is the sum of its values, divided by the number of nodes.)
+
+Example 1:
+
+Input: [5,6,1]
+Output: 6.00000
+Explanation: 
+For the node with value = 5 we have an average of (5 + 6 + 1) / 3 = 4.
+For the node with value = 6 we have an average of 6 / 1 = 6.
+For the node with value = 1 we have an average of 1 / 1 = 1.
+So the answer is 6 which is the maximum.
+
+Note:
+
+The number of nodes in the tree is between 1 and 5000.
+Each node will have a value between 0 and 100000.
+Answers will be accepted as correct if they are within 10^-5 of the correct answer.
+
+*/
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number}
+ */
+const maximumAverageSubtree = function(root) {
+  let max = -Number.MIN_VALUE;
+  function helper(root) {
+    if (!root) return [0, 0]; // [value, number of nodes]
+    const [lTotal, lNum] = helper(root.left);
+    const [rTotal, rNum] = helper(root.right);
+    max = Math.max(max, (rTotal + lTotal + root.val) / (rNum + lNum + 1));
+    return [lTotal + rTotal + root.val, lNum + rNum + 1];
+  }
+  helper(root);
+  return max;
+};