add threeSum.cpp

Author: kamyu104

C++/threeSum.cpp

@@ -0,0 +1,47 @@
+// Time Complexity: O(n^2)
+// Space Complexity: O(1)
+
+class Solution {
+    public:
+        vector<vector<int> > threeSum(vector<int> &num) {
+            vector<vector<int> > ans;
+            const int target = 0;
+
+            if(num.size() < 3)
+                return ans;
+
+            sort(num.begin(), num.end());
+            auto last = num.end();
+            for(auto a = num.begin(); a < prev(last, 2); ++a) {
+                if(a > num.begin() && *a == *(a - 1))
+                    continue;
+                auto b = next(a);
+                auto c = prev(last);
+
+                while(b < c) {
+                    if(b > next(a) && *b == *(b - 1)) {
+                        ++b;
+                    }
+                    else if(c < prev(last) && *c == *(c + 1)) { 
+                        --c;
+                    }
+                    else {
+                        const int sum = *a + *b + *c;
+
+                        if(sum < target)
+                            ++b;
+                        else if(sum > target)
+                            --c;
+                        else {
+                            ans.push_back({ *a, *b, *c});
+                            ++b;
+                            --c;
+                        }
+                    }
+                }
+            }
+
+            return ans;
+        }
+};
+

C++/threeSum2.cpp

@@ -0,0 +1,44 @@
+// Time Complexity: O(n^2)
+// Space Complexity: O(1)
+
+class Solution {
+    public:
+        vector<vector<int> > threeSum(vector<int> &num) {
+            vector<vector<int> > ans;
+            const int target = 0;
+
+            sort(num.begin(), num.end());
+            auto last = num.rend();
+            for(auto a = num.rbegin(); a < prev(last, 2); ++a) {
+                if(a > num.rbegin() && *a == *(a - 1))
+                    continue;
+                auto b = next(a);
+                auto c = prev(last);
+
+                while(b < c) {
+                    if(b > next(a) && *b == *(b - 1)) {
+                        ++b;
+                    }
+                    else if(c < prev(last) && *c == *(c + 1)) { 
+                        --c;
+                    }
+                    else {
+                        const int sum = *a + *b + *c;
+
+                        if(sum < target)
+                            --c;
+                        else if(sum > target)
+                            ++b;
+                        else {
+                            ans.push_back({ *c, *b, *a});
+                            ++b;
+                            --c;
+                        }
+                    }
+                }
+            }
+
+            return ans;
+        }
+};
+