Added solution for "combination sum" problem(Leetcode 39)

combination_sum.cpp

@@ -0,0 +1,79 @@
+
+// Problem Link: https://leetcode.com/problems/combination-sum/
+
+class Solution
+{
+public:
+    bool helper(vector<int> &nums, int currSum, int target, unordered_map<int, int> &perm, vector<unordered_map<int, int>> &collection, int index)
+    {
+        if (currSum > target)
+        {
+            return false;
+        }
+
+        if (currSum == target)
+        {
+            bool found = false;
+            for (auto m : collection)
+            {
+                if (m == perm)
+                {
+                    found = true;
+                    break;
+                }
+            }
+            if (!found)
+            {
+                collection.push_back(perm);
+            }
+            return true;
+        }
+
+        for (int i = index; i < nums.size(); i++)
+        {
+            perm[nums[i]] += 1;
+            currSum += nums[i];
+            bool flag = helper(nums, currSum, target, perm, collection, i);
+            perm[nums[i]] -= 1;
+            if (perm[nums[i]] <= 0)
+            {
+                perm.erase(nums[i]);
+            }
+            currSum -= nums[i];
+            if (!flag)
+            {
+                break;
+            }
+        }
+
+        return true;
+    }
+
+    vector<vector<int>> combinationSum(vector<int> &candidates, int target)
+    {
+        unordered_map<int, int> perm;
+        vector<unordered_map<int, int>> collection;
+
+        sort(candidates.begin(), candidates.end());
+        helper(candidates, 0, target, perm, collection, 0);
+
+        vector<vector<int>> allPerms;
+
+        for (auto m : collection)
+        {
+            vector<int> p;
+            for (auto it : m)
+            {
+                int num = it.first;
+                int freq = it.second;
+                while (freq--)
+                {
+                    p.push_back(num);
+                }
+            }
+            allPerms.push_back(p);
+        }
+
+        return allPerms;
+    }
+};