3Sum

Author: ssfxz

15.cpp

@@ -0,0 +1,37 @@
+// Time:  O(n^2)
+// Space: O(1)
+
+class Solution {
+public:
+    vector<vector<int>> threeSum(vector<int>& nums) {
+        vector<vector<int>> ans;
+        const int target = 0;
+
+        // Make nums in increasing order. Time: O(nlogn)
+        sort(nums.begin(), nums.end());
+
+        for (int i = 0; i < nums.size() - 2; ++i) {
+            if (i == 0 || nums[i] != nums[i - 1]) {  // Skip duplicated.
+                for (int j = i + 1, k = nums.size() - 1; j < k; ) {  // Time: O(n) for each i.
+                    if (j - 1 > i  && nums[j] == nums[j - 1]) {  // Skip duplicated.
+                        ++j;
+                    } else if (k + 1 < nums.size() && nums[k] == nums[k + 1]) {  // Skip duplicated.
+                        --k;
+                    } else {
+                        const auto sum = nums[i] + nums[j] + nums[k];
+                        if (sum > target) {  // Should decrease sum.
+                            --k;
+                        } else if (sum < target) {  // Should increase sum.
+                            ++j;
+                        } else {
+                            ans.push_back({nums[i], nums[j], nums[k]});
+                            ++j, --k;
+                        }
+                    }
+                }
+            }
+        }
+
+        return ans;
+    }
+};