Add some solution

leetcode/problems/java/combination-sum-ii.java

@@ -0,0 +1,29 @@
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Arrays;
+
+class Solution {
+  public List<List<Integer>> combinationSum2(int[] candidates, int target) {
+    List<List<Integer>> result = new ArrayList<>();
+    Arrays.sort(candidates);
+    backtrack(result, new ArrayList<>(), candidates, target, 0);
+    return result;
+  }
+
+  void backtrack(List<List<Integer>> result, List<Integer> tempList, int[] candidates, int remain, int start) {
+    if (remain < 0) {
+      return;
+    } else if (remain == 0) {
+      result.add(new ArrayList<>(tempList));
+    } else {
+      for (int i = start; i < candidates.length; i++) {
+        if (i > start && candidates[i] == candidates[i - 1]) {
+          continue;
+        }
+        tempList.add(candidates[i]);
+        backtrack(result, tempList, candidates, remain - candidates[i], i + 1);
+        tempList.remove(tempList.size() - 1);
+      }
+    }
+  }
+}

leetcode/problems/java/combination-sum.java

@@ -0,0 +1,27 @@
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Arrays;
+
+class Solution {
+
+    public List<List<Integer>> combinationSum(int[] nums, int target) {
+        List<List<Integer>> list = new ArrayList<>();
+        Arrays.sort(nums);
+        backtrack(list, new ArrayList<>(), nums, target, 0);
+        return list;
+    }
+
+    private void backtrack(List<List<Integer>> list, List<Integer> tempList, int[] nums, int remain, int start) {
+        if (remain < 0)
+            return;
+        else if (remain == 0)
+            list.add(new ArrayList<>(tempList));
+        else {
+            for (int i = start; i < nums.length; i++) {
+                tempList.add(nums[i]);
+                backtrack(list, tempList, nums, remain - nums[i], i);
+                tempList.remove(tempList.size() - 1);
+            }
+        }
+    }
+}

leetcode/problems/java/letter-combinations-of-a-phone-number.java

@@ -0,0 +1,28 @@
+import java.util.List;
+import java.util.ArrayList;
+
+class Solution {
+  private String[] map = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
+
+  public List<String> letterCombinations(String digits) {
+    List<String> res = new ArrayList<>();
+    if (digits.length() == 0) {
+      return res;
+    }
+    backtrack(digits, 0, new StringBuilder(), res);
+    return res;
+  }
+
+  private void backtrack(String s, int start, StringBuilder sb, List<String> res) {
+    if (start == s.length()) {
+      res.add(sb.toString());
+      return;
+    }
+    String letters = map[s.charAt(start) - '0'];
+    for (int i = 0; i < letters.length(); i++) {
+      sb.append(letters.charAt(i));
+      backtrack(s, start + 1, sb, res);
+      sb.deleteCharAt(sb.length() - 1);
+    }
+  }
+}

leetcode/problems/java/sum-of-all-subset-xor-totals.java

@@ -0,0 +1,23 @@
+class Solution {
+
+  public int subsetXORSum(int[] nums) {
+    return backtrack(nums, 0, 0);
+  }
+
+  int backtrack(int[] nums, int start, int xor) {
+    if (start >= nums.length)
+      return xor;
+    return backtrack(nums, start + 1, xor) + backtrack(nums, start + 1, xor ^ nums[start]);
+  }
+}
+
+class Solution2 {
+  public int subsetXORSum(int[] nums) {
+    int sum = 0;
+    int n = nums.length;
+    for (int num : nums) {
+      sum |= num;
+    }
+    return sum * (1 << (n - 1));
+  }
+}