Permutation of array

Question

For example I have this array:

int a[] = new int[]{3,4,6,2,1};

I need list of all permutations such that if one is like this, {3,2,1

Answer 1

A simple java implementation, refer to c++ std::next_permutation:

public static void main(String[] args){
    int[] list = {1,2,3,4,5};
    List> output = new Main().permute(list);
    for(List result: output){
        System.out.println(result);
    }

}

public List> permute(int[] nums) {
    List> list = new ArrayList>();
    int size = factorial(nums.length);

    // add the original one to the list
    List seq = new ArrayList();
    for(int a:nums){
        seq.add(a);
    }
    list.add(seq);

    // generate the next and next permutation and add them to list
    for(int i = 0;i < size - 1;i++){
        seq = new ArrayList();
        nextPermutation(nums);
        for(int a:nums){
            seq.add(a);
        }
        list.add(seq);
    }
    return list;
}


int factorial(int n){
    return (n==1)?1:n*factorial(n-1);
}

void nextPermutation(int[] nums){
    int i = nums.length -1; // start from the end

    while(i > 0 && nums[i-1] >= nums[i]){
        i--;
    }

    if(i==0){
        reverse(nums,0,nums.length -1 );
    }else{

        // found the first one not in order 
        int j = i;

        // found just bigger one
        while(j < nums.length && nums[j] > nums[i-1]){
            j++;
        }
        //swap(nums[i-1],nums[j-1]);
        int tmp = nums[i-1];
        nums[i-1] = nums[j-1];
        nums[j-1] = tmp;
        reverse(nums,i,nums.length-1);  
    }
}

// reverse the sequence
void reverse(int[] arr,int start, int end){
    int tmp;
    for(int i = 0; i <= (end - start)/2; i++ ){
        tmp = arr[start + i];
        arr[start + i] = arr[end - i];
        arr[end - i ] = tmp;
    }
}