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LeetCode Backtracking Problems

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LeetCode 78: Subsets

Question

Given a set of distinct integers, nums, return all possible subsets.

Note: The solution set must not contain duplicate subsets.

For example, If nums = [1,2,3], a solution is: 

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[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

Explanation

回溯递归

Code

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public List<List<Integer>> subsets(int[] nums) {
    List<List<Integer>> list = new ArrayList<>();
    Arrays.sort(nums);
    subsetsHelper(list, new ArrayList<>(),nums, 0);
    return list;
}
public void subsetsHelper(List<List<Integer>> list, List<Integer> tmpList, int[] nums, int start) {
    System.out.println(tmpList.toString());
    list.add(new ArrayList<>(tmpList));
    for (int i=start; i<nums.length; i++) {
        System.out.println(i);
        tmpList.add(nums[i]);
        subsetsHelper(list, tmpList, nums, i+1);
        tmpList.remove(tmpList.size()-1);
}

LeetCode 90. Subsets II

Question

Given a collection of integers that might contain duplicates, nums, return all possible subsets.

Note: The solution set must not contain duplicate subsets.

For example, If nums = [1,2,2], a solution is: 

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[
  [2],
  [1],
  [1,2,2],
  [2,2],
  [1,2],
  []
]

Explanation

和上一题一致，只是对于重复的，skip掉。

Code

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public List<List<Integer>> subsetsWithDup(int[] nums) {
    List<List<Integer>> list = new ArrayList<>();
    Arrays.sort(nums);
    subsetsHelper(list, new ArrayList<>(),nums, 0);
    return list;
}
public void subsetsHelper(List<List<Integer>> list, List<Integer> tmpList, int[] nums, int start) {
    System.out.println(tmpList.toString());
    list.add(new ArrayList<>(tmpList));
    for (int i=start; i<nums.length; i++) {
        if (i != start && nums[i] == nums[i-1] ) continue;
        tmpList.add(nums[i]);
        subsetsHelper(list, tmpList, nums, i+1);
        tmpList.remove(tmpList.size()-1);
    }
}

LeeCode 46. Permutations

Question

Given a collection of distinct numbers, return all possible permutations.

For example, [1,2,3] have the following permutations: 

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[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

Explanation

Code

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public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> list = new ArrayList<>();
        subsetsHelper(list, new ArrayList<>(),nums);
        return list;
    }
    public void subsetsHelper(List<List<Integer>> list, List<Integer> tmpList, int[] nums) {
        if (tmpList.size() == nums.length) {
            list.add(new ArrayList<>(tmpList));
        }
        else {
            for (int i= 0; i<nums.length; i++) {
                if (tmpList.contains(nums[i])) continue;
                tmpList.add(nums[i]);
                subsetsHelper(list, tmpList, nums);
                tmpList.remove(tmpList.size()-1);
            }
        }
}