Add solution #2400

JoshCrozier · JoshCrozier · commit 1661e0a7536f · 2025-08-16T08:21:14.000-05:00
diff --git a/README.md b/README.md
@@ -2176,6 +2176,7 @@
 2397|[Maximum Rows Covered by Columns](./solutions/2397-maximum-rows-covered-by-columns.js)|Medium|
 2398|[Maximum Number of Robots Within Budget](./solutions/2398-maximum-number-of-robots-within-budget.js)|Hard|
 2399|[Check Distances Between Same Letters](./solutions/2399-check-distances-between-same-letters.js)|Easy|
+2400|[Number of Ways to Reach a Position After Exactly k Steps](./solutions/2400-number-of-ways-to-reach-a-position-after-exactly-k-steps.js)|Medium|
 2401|[Longest Nice Subarray](./solutions/2401-longest-nice-subarray.js)|Medium|
 2402|[Meeting Rooms III](./solutions/2402-meeting-rooms-iii.js)|Hard|
 2403|[Minimum Time to Kill All Monsters](./solutions/2403-minimum-time-to-kill-all-monsters.js)|Hard|
diff --git a/solutions/2400-number-of-ways-to-reach-a-position-after-exactly-k-steps.js b/solutions/2400-number-of-ways-to-reach-a-position-after-exactly-k-steps.js
@@ -0,0 +1,49 @@
+/**
+ * 2400. Number of Ways to Reach a Position After Exactly k Steps
+ * https://leetcode.com/problems/number-of-ways-to-reach-a-position-after-exactly-k-steps/
+ * Difficulty: Medium
+ *
+ * You are given two positive integers startPos and endPos. Initially, you are standing at
+ * position startPos on an infinite number line. With one step, you can move either one position
+ * to the left, or one position to the right.
+ *
+ * Given a positive integer k, return the number of different ways to reach the position endPos
+ * starting from startPos, such that you perform exactly k steps. Since the answer may be very
+ * large, return it modulo 109 + 7.
+ *
+ * Two ways are considered different if the order of the steps made is not exactly the same.
+ *
+ * Note that the number line includes negative integers.
+ */
+
+/**
+ * @param {number} startPos
+ * @param {number} endPos
+ * @param {number} k
+ * @return {number}
+ */
+var numberOfWays = function(startPos, endPos, k) {
+  const MOD = 1e9 + 7;
+  const dp = new Array(3000).fill().map(() => new Array(k + 1).fill(-1));
+
+  return helper(startPos, endPos, k);
+
+  function helper(currentPos, targetPos, remainingSteps) {
+    if (currentPos === targetPos && remainingSteps === 0) {
+      return 1;
+    }
+    if (remainingSteps === 0) {
+      return 0;
+    }
+
+    const dpIndex = currentPos + 1000;
+    if (dp[dpIndex][remainingSteps] !== -1) {
+      return dp[dpIndex][remainingSteps];
+    }
+    const leftMove = helper(currentPos - 1, targetPos, remainingSteps - 1);
+    const rightMove = helper(currentPos + 1, targetPos, remainingSteps - 1);
+
+    dp[dpIndex][remainingSteps] = (leftMove + rightMove) % MOD;
+    return dp[dpIndex][remainingSteps];
+  }
+};
