feat : combination-sum

Author: ekgns33

combination-sum/ekgns33.java

@@ -0,0 +1,42 @@
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ input : array of distinct integers, single integer target
+ output : all unique combinations of candidates where chosen ones sum is target
+ constraints:
+ 1) can we use same integer multiple times?
+ yes
+ 2) input array can be empty?
+ no. [1, 30]
+
+
+ solution 1)
+ combination >> back-tracking
+ O(2^n) at most 2^30 << (2^10)^3 ~= 10^9
+
+ tc : O(2^n) sc : O(n) call stack
+ */
+class Solution {
+  public List<List<Integer>> combinationSum(int[] candidates, int target) {
+    List<List<Integer>> answer = new ArrayList<>();
+    List<Integer> prev = new ArrayList<>();
+    backTrackingHelper(answer, prev, candidates, target, 0, 0);
+    return answer;
+  }
+  private void backTrackingHelper(List<List<Integer>> ans, List<Integer> prev, int[] cands, int target, int curSum, int p) {
+    if(curSum == target) {
+      ans.add(new ArrayList(prev));
+      return;
+    }
+    if(p >= cands.length) return;
+
+    for(int i = p; i< cands.length; i++) {
+      if((curSum + cands[i]) <= target) {
+        prev.add(cands[i]);
+        backTrackingHelper(ans, prev, cands, target, curSum + cands[i],i);
+        prev.remove(prev.size() - 1);
+      }
+    }
+  }
+}