leetcode new problems

leetcode/lc18/ksum.cpp

@@ -0,0 +1,45 @@
+class Solution{
+    public:
+        vector<vector<int>> foursum(vector<int> &nums, int target){
+            sort(begin(nums), end(nums));
+            return ksum(nums, target, 0 ,4);
+        }
+        vector<vector<int>> ksum(vector<int> &nums, long long target, int start, int k){
+            vector<vector<int>> res;
+            if(start == nums.size()) return res;
+            long long avg = target/k;
+            if(nums[start] > avg || nums.back()<avg) return res;
+
+            if(k==2) return twosum(nums, target, start);
+
+            for(int i = start; i<nums.size(); i++){
+                if(i == start || nums[i]!= nums[i-1]){
+                    for(auto &subset : ksum(nums, target - nums[i], i+1, k-1)){
+                        res.push_back({nums[i]});
+                        res.back().insert(end(res.back()), begin(subset), end(subset));
+                    }
+                }
+            }
+            return res;
+
+            
+        }
+        vector<vector<int>> twosum(vector<int> &nums, long long target, int start){
+            vector<vector<int>> res;
+            unordered_set<long long> s;
+            for(int i = start; i<nums.size();i++){
+                if(res.empty() || res.back()[1]!= nums[i]){
+                    if(s.count(target- nums[i])){
+                        res.push_back({int(target-nums[i]),nums[i]});
+                    }
+                }
+                 s.insert(nums[i]);
+
+            }
+        return res;        
+    }
+        
+        
+};
+// TC : o(n^3) or o(n^(k-1)) in general
+// SC : O(n)