[LeetCode] 46. Permutations 全排列

Given a collection of numbers, return all possible permutations.

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

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

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

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

给一个没有重复的数组，返回全部的排列可能。

解法：递归Backtracking

Java:

public List<List<Integer>> permute(int[] nums) {
   List<List<Integer>> list = new ArrayList<>();
   // Arrays.sort(nums); // not necessary
   backtrack(list, new ArrayList<>(), nums);
   return list;
}

private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums){
   if(tempList.size() == nums.length){
      list.add(new ArrayList<>(tempList));
   } else{
      for(int i = 0; i < nums.length; i++){ 
         if(tempList.contains(nums[i])) continue; // element already exists, skip
         tempList.add(nums[i]);
         backtrack(list, tempList, nums);
         tempList.remove(tempList.size() - 1);
      }
   }
} 

Python: Recursion

class Solution:
    # @param num, a list of integer
    # @return a list of lists of integers
    def permute(self, num):
        result = []
        used = [False] * len(num)
        self.permuteRecu(result, used, [], num)
        return result
    
    def permuteRecu(self, result, used, cur, num):
        if len(cur) == len(num):
            result.append(cur[:])
            return
        for i in xrange(len(num)):
            if not used[i]:
                used[i] = True
                cur.append(num[i])
                self.permuteRecu(result, used, cur, num)
                cur.pop()
                used[i] = False

C++: Recursion

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        vector<vector<int> > res;
        permuteDFS(num, 0, res);
        return res;
    }
    void permuteDFS(vector<int> &num, int start, vector<vector<int> > &res) {
        if (start >= num.size()) res.push_back(num);
        for (int i = start; i < num.size(); ++i) {
            swap(num[start], num[i]);
            permuteDFS(num, start + 1, res);
            swap(num[start], num[i]);
        }
    }
};

C++: Recursion

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        vector<vector<int> > res;
        vector<int> out;
        vector<int> visited(num.size(), 0);
        permuteDFS(num, 0, visited, out, res);
        return res;
    }
    void permuteDFS(vector<int> &num, int level, vector<int> &visited, vector<int> &out, vector<vector<int> > &res) {
        if (level == num.size()) res.push_back(out);
        else {
            for (int i = 0; i < num.size(); ++i) {
                if (visited[i] == 0) {
                    visited[i] = 1;
                    out.push_back(num[i]);
                    permuteDFS(num, level + 1, visited, out, res);
                    out.pop_back();
                    visited[i] = 0;
                }
            }
        }
    }
};

　　

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