add js solution for leetcode 21 344 387

Author: swiftwind0405

README.md

@@ -1,7 +1,6 @@
-LeetCode
-========
+# LeetCode
 
-### LeetCode Algorithm
+## LeetCode Algorithm
 
 | # | Title | Solution | Difficulty |
 |---| ----- | -------- | ---------- |
@@ -116,7 +115,7 @@ LeetCode
 |390|[Elimination Game](https://leetcode.com/problems/elimination-game/) | |Medium|
 |389|[Find the Difference](https://leetcode.com/problems/find-the-difference/) | |Easy|
 |388|[Longest Absolute File Path](https://leetcode.com/problems/longest-absolute-file-path/) | |Medium|
-|387|[First Unique Character in a String](https://leetcode.com/problems/first-unique-character-in-a-string/) | |Easy|
+|387|[First Unique Character in a String](https://leetcode.com/problems/first-unique-character-in-a-string/) | [js](./algorithms/firstUniqueCharacterInAString/firstUniqueCharacterInAString.js) |Easy|
 |386|[Lexicographical Numbers](https://leetcode.com/problems/lexicographical-numbers/) | |Medium|
 |385|[Mini Parser](https://leetcode.com/problems/mini-parser/) | |Medium|
 |384|[Shuffle an Array](https://leetcode.com/problems/shuffle-an-array/) | |Medium|
@@ -133,7 +132,7 @@ LeetCode
 |349|[Intersection of Two Arrays](https://leetcode.com/problems/intersection-of-two-arrays/) | [js](./algorithms/intersectionOfTwoArrays/intersectionOfTwoArrays.js) |Easy|
 |347|[Top K Frequent Elements](https://leetcode.com/problems/top-k-frequent-elements/) | |Medium|
 |345|[Reverse Vowels of a String](https://leetcode.com/problems/reverse-vowels-of-a-string/) | |Easy|
-|344|[Reverse String](https://leetcode.com/problems/reverse-string/) [java](./algorithms/resverseString/Solution.java) | |Easy|
+|344|[Reverse String](https://leetcode.com/problems/reverse-string/) [java](./algorithms/resverseString/Solution.java), [js](./algorithms/resverseString/resverseString.js) | |Easy|
 |343|[Integer Break](https://leetcode.com/problems/integer-break/) | |Medium|
 |342|[Power of Four](https://leetcode.com/problems/power-of-four/) | |Easy|
 |341|[Flatten Nested List Iterator](https://leetcode.com/problems/flatten-nested-list-iterator/) | |Medium|
@@ -396,7 +395,7 @@ LeetCode
 |24|[Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/)| [js](./algorithms/swapNodesInPairs/swapNodesInPairs.js) |Medium|
 |23|[Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)| |Hard|
 |22|[Generate Parentheses](https://leetcode.com/problems/generate-parentheses/)| [js](./algorithms/generateParentheses/generateParentheses.js) |Medium|
-|21|[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)| [java](./algorithm/mergeTwoSortedLists/Solution.java) |Easy|
+|21|[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)| [java](./algorithm/mergeTwoSortedLists/Solution.java), [js](./algorithms/mergeTwoSortedLists/mergeTwoSortedLists.js) |Easy|
 |20|[Valid Parentheses](https://leetcode.com/problems/valid-parentheses/)| [js](./algorithms/validParentheses/validParentheses.js) |Easy|
 |19|[Remove Nth Node From End of List](https://leetcode.com/problems/remove-nth-node-from-end-of-list/)| [js](./algorithms/removeNthNodeFromEndOfList/removeNthNodeFromEndOfList.js) |Easy|
 |18|[4Sum](https://leetcode.com/problems/4sum/)| [js](./algorithms/4Sum/4Sum.js) |Medium|

algorithms/firstUniqueCharacterInAString/firstUniqueCharacterInAString.js

@@ -0,0 +1,16 @@
+var firstUniqChar = function(s) {
+    const startIndex = 'a'.charCodeAt();
+    // 创建一个长度为26的字母表，初始频率都为0
+    const freq = Array.from({length: 26}, x => 0);
+    // 与a做比较得到索引位置，如果重复则频率+1
+    for (let i = 0; i < s.length; i++) {
+        freq[s.charCodeAt(i) - startIndex] ++;
+    }
+    // 最后再扫一遍s，如果在频率表中的频率为1，则代表只出现过一次
+    for (let i = 0; i < s.length; i++) {
+        if (freq[s.charCodeAt(i) - startIndex] === 1) {
+            return i;
+        }
+    }
+    return -1;
+};

algorithms/mergeTwoSortedLists/mergeTwoSortedLists.js

@@ -0,0 +1,19 @@
+var mergeTwoLists = function(l1, l2) {
+    const prehead = new ListNode(-1);
+    let prev = prehead;
+    while (l1 !== null && l2 !== null) {
+        // l1和l2谁小就接在后面，同时当前指针后移一位
+        if (l1.val <= l2.val) {
+            prev.next = l1;
+            l1 = l1.next;
+        } else {
+            prev.next = l2;
+            l2 = l2.next;
+        }
+        // 不管将哪一个元素接在了后面，我都需要把 prev 向后移一位。
+        prev = prev.next;
+    }
+    // 合并后 l1 和 l2 最多只有一个还未被合并完，直接将链表末尾指向未合并完的链表即可
+    prev.next = l1 === null ? l2 : l1;
+    return prehead.next;
+};

algorithms/reverseString/resverseString.js

@@ -0,0 +1,10 @@
+var reverseString = function(s) {
+    const len = s.length;
+    const halfLen = Math.round(len/2);
+    // 只用遍历前一半数组
+    for (let i = 0; i < halfLen; i++) {
+        // 两两交换
+        [s[i], s[len - 1 -i]] = [s[len - 1 -i], s[i]];
+    }
+    return s;
+};