47. Permutations II
Tags: ‘Backtracking’
Given a collection of numbers that might contain duplicates, return all possible unique permutations.
Example:
Input: [1,1,2] Output: [ [1,1,2], [1,2,1], [2,1,1] ]
Solution
标准的backtracking 模板
public List<List<Integer>> permuteUnique(int[] nums) {
List<List<Integer>> result = new LinkedList<>();
Arrays.sort(nums);
backtrack(nums, new LinkedList<>(), result, new boolean[nums.length]);
return result;
}
private void backtrack(int[] nums, List<Integer> temp, List<List<Integer>> result, boolean[] used) {
if (temp.size() == nums.length) {
result.add(new ArrayList<>(temp));
return;
}
for (int i = 0; i < nums.length; i++) {
//剪枝 after sort only use first appearance
if (used[i] || (i > 0 && nums[i] == nums[i-1] && !used[i-1])) {
continue;
}
used[i] = true;
temp.add(nums[i]);
backtrack(nums, temp, result, used);
used[i] = false;
temp.remove(temp.size() - 1);
}
}