Merge pull request #36 from rachitkewl/patch-3

Mock Coding Interview - Combination Sum Problem

Author: rachitiitr

LeetCode/39.combination-sum.cpp

@@ -0,0 +1,32 @@
+using choice = vector<int>;
+vector<int> arr = {};
+
+// returning all possible choices to make target sum by using suffix of array [curIndex, ...]
+vector<choice> getAllChoices(int curIndex, int target) {
+    // base case
+    if(target < 0) return {}; // no valid choice
+    if(target == 0) return {{}}; // one choice, and you chose nothing
+    if(curIndex == arr.size()) return {};
+    
+    int curNumber = arr[curIndex];
+    
+    vector<choice> ans = getAllChoices(curIndex+1, target); // curNumber is not used at all
+    
+    vector<choice> other = getAllChoices(curIndex, target - curNumber); // using it once
+    for(choice c: other) {
+        c.push_back(curNumber);
+        // now c is a valid choice 
+        ans.push_back(c);
+    }
+    
+    return ans;
+}
+
+class Solution {
+    
+public:
+    vector<choice> combinationSum(vector<int>& candidates, int target) {
+        arr = candidates;
+        return getAllChoices(0, target);
+    }
+};