java: add «40. Combination Sum II»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/combination-sum-ii.java

@@ -0,0 +1,41 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.List;
+
+class Solution {
+  int[] candidates;
+  private HashSet<List<Integer>> combinations;
+
+  private void combinationSum2(ArrayList<Integer> combination, int target, int i0) {
+    if (target == 0) {
+      combinations.add(combination.stream().toList());
+      return;
+    }
+
+    if (i0 >= candidates.length) {
+      return;
+    }
+
+    for (int i = i0; i < candidates.length && candidates[i] <= target; ++i) {
+      if (i > i0 && candidates[i] == candidates[i - 1]) {
+        continue;
+      }
+
+      combination.addLast(candidates[i]);
+      combinationSum2(combination, target - candidates[i], i + 1);
+      combination.removeLast();
+    }
+  }
+
+  public List<List<Integer>> combinationSum2(int[] candidates, int target) {
+    this.candidates = candidates;
+    combinations = new HashSet<>();
+
+    Arrays.sort(this.candidates);
+    combinationSum2(new ArrayList<>(), target, 0);
+    var result = combinations.stream().toList();
+
+    return result;
+  }
+}