function中的引用传递和不引用传递

Question

i was solving this problem: https://leetcode.com/problems/permutations-ii/

here is my working solution:

class Solution {
public:
vector<vector<int>> output;
void swap(vector<int>& nums, int index, int i){
    int temp=nums[index];
    nums[index]=nums[i];
    nums[i]=temp;
    return;
}
void helper(vector<int> nums, int index){ //here it is normally passed
    if(index==nums.size()){
        output.push_back(nums);
        return;
    }
    for(int i=index; i<nums.size(); i++){
        if(i!=index && nums[i]==nums[index]){continue;}
        swap(nums, i, index);
        helper(nums, index+1);
        // swap(nums, i, index);         //commented and used only with passed by reference
    }
}
vector<vector<int>> permuteUnique(vector<int>& nums) {
    int size = nums.size();
    sort(begin(nums) ,end(nums));
    helper(nums, 0);
    return output;
}
};

WORKING example

and then this is the solution in which i am passing by reference and swapping while backtracking:

class Solution {
public:
vector<vector<int>> output;
void swap(vector<int>& nums, int index, int i){
    int temp=nums[index];
    nums[index]=nums[i];
    nums[i]=temp;
    return;
}
void helper(vector<int> &nums, int index){ //here nums is passed by reference
    if(index==nums.size()){
        output.push_back(nums);
        return;
    }
    for(int i=index; i<nums.size(); i++){
        if(i!=index && nums[i]==nums[index]){continue;}
        swap(nums, i, index);
        helper(nums, index+1);
        swap(nums, i, index);             //backtrack
    }
}
vector<vector<int>> permuteUnique(vector<int>& nums) {
    int size = nums.size();
    sort(begin(nums) ,end(nums));
    helper(nums, 0);
    return output;
}
};

NOT WORKING Example

But this solution does not apppear to be working fine. Can anyone please explain why there is a difference between the two?

Answer 1

In short - when you pass by reference, changes you make to the object inside your helper are applied to the original object. When you pass a copy ( like the original solution did) any changes you make will be applied to the copy and not the original.

In your solution, you pass a reference to a vector into the helper function, inside of which - you call push_back() on that reference - which means the original vector gets bigger, and this is why your solution gives a result which is longer than expected.