add solution for leetcode 18

Author: swiftwind0405

README.md

@@ -394,7 +394,7 @@ LeetCode
 |21|[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)| [java](./algorithm/mergeTwoSortedLists/Solution.java) |Easy|
 |20|[Valid Parentheses](https://leetcode.com/problems/valid-parentheses/)| |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/)| |Medium|
+|18|[4Sum](https://leetcode.com/problems/4sum/)| [js](./algorithms/4Sum/4Sum.js) |Medium|
 |17|[Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)| |Medium|
 |16|[3Sum Closest](https://leetcode.com/problems/3sum-closest/)| |Medium|
 |15|[3Sum](https://leetcode.com/problems/3sum/)| [js](./algorithms/3Sum/3Sum.js) |Medium|

algorithms/4Sum/4Sum.js

@@ -0,0 +1,36 @@
+var fourSum = function(nums, target) {
+    const res = [];
+    const len = nums.length;
+    if (nums == null || len < 4) return res;
+
+    nums = nums.sort((a, b) => a - b);
+
+    for (let i = 0; i < len - 3; i++) {
+        //计算当前的最小值，如果最小值都比target大，或者最大值都比target小不用再继续计算了
+        if (nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target || 
+                nums[len - 1] + nums[len - 2] + nums[len - 3] + nums[len -4] < target) break;
+        if (i === 0 || nums[i] !== nums[i-1]) {
+            for (let j = i + 1; j < len - 2; j++) {
+                if (j === i + 1 || nums[j] !== nums[j - 1]) {
+                    let R = len - 1, L = j + 1;
+                    //计算当前的最小值，如果最小值都比target大，不用再继续计算了
+                    if (nums[i] + nums[j] + nums[L] + nums[L + 1] > target) continue;
+                    //计算当前最大值，如果最大值都比target小，不用再继续计算了
+                    if (nums[i] + nums[j] + nums[R] + nums[R - 1] < target) continue;
+                    while (L < R) {
+                        let sum = nums[i] + nums[j] + nums[L] + nums[R];
+                        if (sum == target) {
+                            res.push([nums[i], nums[j], nums[L], nums[R]]);
+                        }
+                        if (sum < target) {
+                            while (nums[L] === nums[++L]);
+                        } else {
+                            while (nums[R] === nums[--R]);
+                        }
+                    }
+                }
+            }
+        }
+    }
+    return res;
+};