Leetcode 1530

README.md

@@ -231,6 +231,7 @@
 - [1323: Maximum 69 number](problems/leetcode/1323)
 - [1356: Sort integers by the number of bits with value 1](problems/leetcode/1356)
 - [1509: Minimum difference between largest and smallest value in three moves](problems/leetcode/1509)
+- [1530: NUmber of Good Leaf Nodes Pairs](problems/leetcode/1530)
 - [1544: Make the string great](problems/leetcode/1544)
 - [1704: Determine if string halves are alike](problems/leetcode/1704)
 - [1706: Where will the ball fall](problems/leetcode/1706)

problems/leetcode/1530/solution.js

@@ -0,0 +1,51 @@
+// https://leetcode.com/problems/number-of-good-leaf-nodes-pairs/
+// solution based on https://leetcode.com/problems/number-of-good-leaf-nodes-pairs/solutions/5493798/ultimate-1-ms-solution-beats-100-java-c-py-js-go-beginner-friendly-explanation/
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @param {number} distance
+ * @return {number}
+ */
+const countPairs = function(root, distance) {
+  let count = 0;
+  const MAX_DISTANCE = 10;
+
+  function dfs(node) {
+    if (!node) return new Array(MAX_DISTANCE + 1).fill(0);
+
+    if (!node.left && !node.right) {
+      // leaf
+      const res = new Array(MAX_DISTANCE + 1).fill(0);
+      res[1] = 1;
+      return res;
+    }
+
+    const left = dfs(node.left);
+    const right = dfs(node.right);
+
+    for (let i = 1; i <= distance; i++) {
+      for (let j = 1; j <= distance - i; j++) {
+        // only leafs will have non-zero values
+        count += left[i] * right[j];
+      }
+    }
+
+    const res = new Array(MAX_DISTANCE + 1).fill(0);
+    for (let i = 1; i < MAX_DISTANCE; i++) {
+      res[i + 1] = left[i] + right[i];
+    }
+
+    return res;
+  }
+
+  dfs(root);
+
+  return count;
+};