Update 4sum.cpp

Author: kamyu104

C++/4sum.cpp

@@ -3,33 +3,31 @@
 
 class Solution {
 public:
-    vector<vector<int> > fourSum(vector<int> &num, int target) {
-        int len = num.size();
-        int left, right, sum;
-        sort(num.begin(), num.end());
-        vector<vector<int>> res;
-        for (int i = 0; i < len - 3; ++i) {
-            if (i && num[i] == num[i - 1]) {
+    vector<vector<int> > fourSum(vector<int> &nums, int target) {
+        sort(begin(nums), end(nums));
+        vector<vector<int>> result;
+        for (int i = 0; i + 3 < size(nums); ++i) {
+            if (i && nums[i] == nums[i - 1]) {
                 continue;
             }
-            for (int j = i + 1; j < len - 2; ++j) {
-                if (j != i + 1 && num[j] == num[j - 1]) {
+            for (int j = i + 1; j + 2 < size(nums); ++j) {
+                if (j != i + 1 && nums[j] == nums[j - 1]) {
                     continue;
                 }
-                sum = target - num[i] - num[j];
-                left = j + 1, right = len - 1;
+                int total = target - nums[i] - nums[j];
+                int left = j + 1, right = size(nums) - 1;
                 while (left < right) {
-                    if (num[left] + num[right] == sum) {
-                        res.push_back({num[i], num[j], num[left], num[right]});
+                    if (nums[left] + nums[right] == total) {
+                        result.push_back({nums[i], nums[j], nums[left], nums[right]});
                         ++left, --right;
-                        while (left < right && num[left] == num[left - 1]) {
+                        while (left < right && nums[left] == nums[left - 1]) {
                             ++left;
                         }
-                        while (left < right && num[right] == num[right + 1]) {
+                        while (left < right && nums[right] == nums[right + 1]) {
                             --right;
                         }
                     } else {
-                        if (num[left] + num[right] > sum) {
+                        if (nums[left] + nums[right] > total) {
                             --right;
                         } else {
                             ++left;
@@ -38,42 +36,6 @@ class Solution {
                 }
             }
         }
-        return res;
-    }
-};
-
-// Time:  O(n^4)
-// Space: O(n^2)
-class Solution2 {
-public:
-    vector<vector<int> > fourSum(vector<int> &num, int target) {
-        vector<vector<int>> ans;
-        if (num.size() < 4)  {
-            return ans;
-        }
-        sort(num.begin(), num.end());
-        unordered_multimap<int, pair<int, int>> cache;
-
-        for (int i = 0; i < num.size(); ++i) {
-            for (int j = i + 1; j < num.size(); ++j) {
-                cache.emplace(num[i] + num[j], make_pair(i, j));
-            }
-        }
-
-        for (auto i = cache.begin(); i != cache.end(); ++i) {
-            auto a = i->second.first;
-            auto b = i->second.second;
-            auto range = cache.equal_range(target - i->first);
-            for (auto j = range.first; j != range.second; ++j) {
-                auto c = j->second.first;
-                auto d = j->second.second;
-                if (b < c) {
-                    ans.push_back({num[a], num[b], num[c], num[d]});
-                }
-            }
-        }
-        sort(ans.begin(), ans.end());
-        ans.erase(unique(ans.begin(), ans.end()), ans.end());
-        return ans;
+        return result;
     }
 };