Why time varies from 20ms to 108ms (with C++ recursive solution)


  • 0
    L

    My code is as followed:

    class Solution {
    public:
        TreeNode* sortedArrayToBST(vector<int>& nums) {
            return sort(nums,0,nums.size());
        }
        TreeNode* sort(vector<int> &nums,int start,int end){
            if(end == start) return NULL;
            int mid = (end+start)/2;
            TreeNode* root = new TreeNode(nums[mid]);
            root->left = sort(nums,start,mid);
            root->right = sort(nums,mid+1,end);
            return root;
        }
    };
    
    

    This code will run in 20ms.
    I just found that if I delete the "&" before nums in the variables declaration part, the time will increase to 108ms. I just cannot understand what's the reason of this.


  • 0
    J

    Without &, nums is passed by value. So each time you call sort(...), nums will be copied. Copy is time consuming.


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