Add solution #903

Author: JoshCrozier

README.md

@@ -713,6 +713,7 @@
 900|[RLE Iterator](./0900-rle-iterator.js)|Medium|
 901|[Online Stock Span](./0901-online-stock-span.js)|Medium|
 902|[Numbers At Most N Given Digit Set](./0902-numbers-at-most-n-given-digit-set.js)|Hard|
+903|[Valid Permutations for DI Sequence](./0903-valid-permutations-for-di-sequence.js)|Hard|
 905|[Sort Array By Parity](./0905-sort-array-by-parity.js)|Easy|
 909|[Snakes and Ladders](./0909-snakes-and-ladders.js)|Medium|
 912|[Sort an Array](./0912-sort-an-array.js)|Medium|

solutions/0903-valid-permutations-for-di-sequence.js

@@ -0,0 +1,50 @@
+/**
+ * 903. Valid Permutations for DI Sequence
+ * https://leetcode.com/problems/valid-permutations-for-di-sequence/
+ * Difficulty: Hard
+ *
+ * You are given a string s of length n where s[i] is either:
+ * - 'D' means decreasing, or
+ * - 'I' means increasing.
+ *
+ * A permutation perm of n + 1 integers of all the integers in the range [0, n] is called a
+ * valid permutation if for all valid i:
+ * - If s[i] == 'D', then perm[i] > perm[i + 1], and
+ * - If s[i] == 'I', then perm[i] < perm[i + 1].
+ *
+ * Return the number of valid permutations perm. Since the answer may be large, return it
+ * modulo 109 + 7.
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var numPermsDISequence = function(s) {
+  const MOD = 1e9 + 7;
+  const n = s.length;
+  const dp = Array.from({ length: n + 1 }, () => new Array(n + 1).fill(0));
+
+  dp[0][0] = 1;
+
+  for (let len = 1; len <= n; len++) {
+    for (let curr = 0; curr <= len; curr++) {
+      if (s[len - 1] === 'D') {
+        for (let prev = curr; prev < len; prev++) {
+          dp[len][curr] = (dp[len][curr] + dp[len - 1][prev]) % MOD;
+        }
+      } else {
+        for (let prev = 0; prev < curr; prev++) {
+          dp[len][curr] = (dp[len][curr] + dp[len - 1][prev]) % MOD;
+        }
+      }
+    }
+  }
+
+  let result = 0;
+  for (let num = 0; num <= n; num++) {
+    result = (result + dp[n][num]) % MOD;
+  }
+
+  return result;
+};