Add Dart solution for 3Sum

dart/three_sum.dart

@@ -0,0 +1,45 @@
+/*
+Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0.
+
+Notice that the solution set must not contain duplicate triplets.
+*/
+
+class Solution {
+  List<List<int>> threeSum(List<int> nums) {
+    nums.sort();
+    final List<List<int>> ans = [];
+
+    for (int i = 0; i < nums.length - 2; i++) {
+      if (i > 0 && nums[i] == nums[i - 1]) {
+        continue;
+      }
+
+      int left = i + 1;
+      int right = nums.length - 1;
+
+      while (left < right) {
+        final int sum = nums[i] + nums[left] + nums[right];
+
+        if (sum == 0) {
+          ans.add([nums[i], nums[left], nums[right]]);
+          left++;
+          right--;
+
+          while (left < right && nums[left] == nums[left - 1]) {
+            left++;
+          }
+
+          while (left < right && nums[right] == nums[right + 1]) {
+            right--;
+          }
+        } else if (sum < 0) {
+          left++;
+        } else {
+          right--;
+        }
+      }
+    }
+
+    return ans;
+  }
+}