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LeetCode 46.Permutations

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Quesiton

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

这道题是排列组合问题，所以用DFS和回溯法递归。注意长度要和原来的数组长度相等。

Code

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