Add JS permutation in string LC medium

Author: nwthomas

src/leetcode/medium/medium.md

@@ -58,6 +58,7 @@
 -   [Number of Islands](number-of-islands)
 -   [Pacific Atlantic Water Flow](pacific-atlantic-water-flow)
 -   [Palindromic Substring](palindromic-substring)
+-   [Permutation in String](permutation-in-string)
 -   [Product of Array Except Self](product-of-array-except-self)
 -   [Remove Nth Node From End of List](remove-nth-node-from-end-of-list)
 -   [Reorder List](reorder-list)

src/leetcode/medium/permutation-in-string/permutationInString.js

@@ -0,0 +1,60 @@
+/*
+https://leetcode.com/problems/permutation-in-string/description
+
+You are given two strings s1 and s2.
+
+Return true if s2 contains a permutation of s1, or false otherwise. That means if a permutation of s1 exists as a substring of s2, then return true.
+
+Both strings only contain lowercase letters.
+
+Example 1:
+Input: s1 = "abc", s2 = "lecabee"
+Output: true
+Explanation: The substring "cab" is a permutation of "abc" and is present in "lecabee".
+
+Example 2:
+Input: s1 = "abc", s2 = "lecaabee"
+Output: false
+
+Constraints:
+1 <= s1.length, s2.length <= 1000
+*/
+
+const checkInclusion = (s1, s2) => {
+    const s1Chars = {};
+    for (const char of s1) {
+        s1Chars[char] = s1Chars[char] ?? 0;
+        s1Chars[char] += 1;
+    }
+
+    let hasPermutation = false;
+    let l = 0;
+    let r = 0;
+
+    while (r < s2.length) {
+        const s2Char = s2[r];
+        if (s1Chars[s2Char]) {
+            s1Chars[s2Char] -= 1;
+
+            if (s1Chars[s2Char] === 0) {
+                delete s1Chars[s2Char];
+            }
+
+            if (Object.keys(s1Chars).length === 0) {
+                hasPermutation = true;
+                break;
+            }
+
+            r += 1;
+        } else {
+            const replacementS2Char = s2[l];
+            s1Chars[replacementS2Char] = s1Chars[replacementS2Char] ?? 0;
+            s1Chars[replacementS2Char] += 1;
+            l += 1;
+        }
+    }
+
+    return hasPermutation;
+};
+
+module.exports = { checkInclusion };

src/leetcode/medium/permutation-in-string/permutationInString.test.js

@@ -0,0 +1,18 @@
+const { checkInclusion } = require("./permutationInString");
+
+describe("checkInclusion", () => {
+    it("returns true if s2 contains a permutation of s1", () => {
+        const result = checkInclusion("abc", "lecabee");
+        expect(result).toBeTruthy();
+    });
+
+    it("returns false if s2 does not contain a permutation of s1", () => {
+        const result = checkInclusion("abc", "lecaabee");
+        expect(result).toBeFalsy();
+    });
+
+    it("correctly returns true if s2 contains a permutation of s1 with multiple characters", () => {
+        const result = checkInclusion("adc", "dadclecabee");
+        expect(result).toBeTruthy();
+    });
+});