added 4 sum same approach as 3 sum

Author: ignacio-chiazzo

18. 4Sum.js

@@ -0,0 +1,41 @@
+/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[][]}
+ */
+var fourSum = function(nums, target) {
+    var results = [];
+    nums.sort(function(a,b) { return a - b });
+    for(var i = 0; i < nums.length - 3; i++) {
+        while(i > 0 && i < nums.length - 3 && nums[i] === nums[i - 1]) {
+            i++;
+        }
+        for(var j = i + 1; j < nums.length - 2; j++) {
+            while(j > i + 1 && j < nums.length - 2 && nums[j] === nums[j - 1]) {
+                j++;
+            }
+            var low = j + 1;
+            var high = nums.length - 1;
+            var newTarget = target - (nums[i] + nums[j]);
+            while(low < high) {
+                partialSum = nums[low] + nums[high];
+                if(partialSum === newTarget) {
+                    results.push([nums[i], nums[j], nums[low], nums[high]]);
+                    high--;
+                    low++;
+                    while(low < high && nums[low] === nums[low - 1]) {
+                        low++;
+                    }
+                    while(low < high && nums[high] === nums[high + 1]) {
+                        high--;
+                    }
+                } else if (partialSum > newTarget) {
+                    high--;
+                } else {
+                    low++;
+                }
+            }
+        }
+    }
+    return results;
+};