Add solution #115

Author: JoshCrozier

Diff for: README.md

@@ -109,6 +109,7 @@
 112|[Path Sum](./0112-path-sum.js)|Easy|
 113|[Path Sum II](./0113-path-sum-ii.js)|Medium|
 114|[Flatten Binary Tree to Linked List](./0114-flatten-binary-tree-to-linked-list.js)|Medium|
+115|[Distinct Subsequences](./0115-distinct-subsequences.js)|Hard|
 116|[Populating Next Right Pointers in Each Node](./0116-populating-next-right-pointers-in-each-node.js)|Medium|
 118|[Pascal's Triangle](./0118-pascals-triangle.js)|Easy|
 119|[Pascal's Triangle II](./0119-pascals-triangle-ii.js)|Easy|

Diff for: solutions/0115-distinct-subsequences.js

@@ -0,0 +1,29 @@
+/**
+ * 115. Distinct Subsequences
+ * https://leetcode.com/problems/distinct-subsequences/
+ * Difficulty: Hard
+ *
+ * Given two strings s and t, return the number of distinct subsequences of s which equals t.
+ *
+ * The test cases are generated so that the answer fits on a 32-bit signed integer.
+ */
+
+/**
+ * @param {string} s
+ * @param {string} t
+ * @return {number}
+ */
+var numDistinct = function(s, t) {
+  const nums = new Array(t.length + 1).fill(0);
+  nums[0] = 1;
+
+  for (let i = 0; i < s.length; i++) {
+    for (let j = nums.length - 1; j >= 0; j--) {
+      if (s[i] === t[j]) {
+        nums[j + 1] += nums[j];
+      }
+    }
+  }
+
+  return nums[t.length];
+};