Signed-off-by: yuduozhou <[email protected]>

Author: yuduozhou

CombinationSum1.java

@@ -0,0 +1,37 @@
+public class Solution {
+    public ArrayList<ArrayList<Integer>> combinationSum(int[] candidates, int target) {
+        // Start typing your Java solution below
+        // DO NOT write main() function
+        Arrays.sort(candidates);
+        //reverse the array
+        for (int i = 0; i < candidates.length/2; i++) {
+            int temp = candidates[i];
+            candidates[i] = candidates[candidates.length-1-i];
+            candidates[candidates.length-1-i] = temp;
+        }
+        return cs (candidates, target, 0);
+    }
+    public ArrayList<ArrayList<Integer>> cs (int[] candidates, int target, int index) {
+        ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
+        if (target == 0) {
+            ret.add(new ArrayList<Integer>());
+            return ret;
+        }
+        if (index >= candidates.length) {
+            return null;
+        }
+        for (int i = 0; candidates[index] * i <= target; i++) {
+            ArrayList<ArrayList<Integer>> r = cs (candidates, target-candidates[index] * i, index + 1);
+            if (r != null) {
+                //add i candidates[index] to all of the arraylists
+                for (int j = 0; j < r.size(); j++) {
+                    for (int k = 0; k < i; k++) {
+                        r.get(j).add(candidates[index]);
+                    }
+                }
+                ret.addAll(r);
+            }
+        }
+        return ret;
+    }
+}

CombinationSum2.java

@@ -0,0 +1,37 @@
+// Thanks to http://heartfire.cc
+public class Solution {
+    public ArrayList<ArrayList<Integer>> combinationSum2(int[] num, int target) {
+        // Start typing your Java solution below
+        // DO NOT write main() function
+        Arrays.sort(num); 
+        int[] backtrack = new int[num.length+1];
+        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
+        backtrack[0] = -1;
+        solve (num, 0, target, backtrack, result, 0);
+        return result;
+    }
+ 
+    public void solve(int[] num, int sum, int target, int[] backtrack, ArrayList<ArrayList<Integer>> result, int n){
+        //backtrack records the indexes in num array
+        //what is my next number's index? start trying from backtrack[n] + 1 to num's end
+        if (target == sum) {
+            //add everything in backtrack to result
+            ArrayList<Integer> ret = new ArrayList<Integer>();
+            for (int i = 1; i <= n; i++) {
+                ret.add(num[backtrack[i]]);
+            }
+            result.add(ret);
+        }
+        if (target < sum) {
+            return;
+        }
+        for (int i = backtrack[n] + 1; i < num.length; i++) {
+            backtrack[n+1] = i; 
+            solve(num, sum+num[i], target, backtrack, result, n+1);
+ 
+            while (i+1 < num.length && num[i+1] == num[i]) {
+                i++;
+            }
+        }
+    }
+}