動機

注意正負數的2倍，分別會變大變小

Problem

Given an array arr of integers, check if there exists two integers N and M such that N is the double of M ( i.e. N = 2 * M).

More formally check if there exists two indices i and j such that :

    t
  • i != j
    • t
  • 0 <= i, j < arr.length
    • t
  • arr[i] == 2 * arr[j]

 

Example 1:

Input: arr = [10,2,5,3]Output: trueExplanation: N = 10 is the double of M = 5,that is, 10 = 2 * 5.

Example 2:

Input: arr = [7,1,14,11]Output: trueExplanation: N = 14 is the double of M = 7,that is, 14 = 2 * 7.

Example 3:

Input: arr = [3,1,7,11]Output: falseExplanation: In this case does not exist N and M, such that N = 2 * M.

 

Constraints:

    t
  • 2 <= arr.length <= 500
    • t
  • -10^3 <= arr[i] <= 10^3

Sol

class Solution {
public:
    bool checkIfExist(vector<int>& arr) {
        sort(arr.begin(),arr.end());
        
        for(auto i=arr.begin();i!=arr.end();i++) {
            int target = (*i < 0) && ((*i) % 2 == 0) ? (*i)/2 : (*i)*2;
            auto j = lower_bound(i+1,arr.end(),target);
                if (j != arr.end() && *j == target) {
                    cout << *i << ',' << *j << '\n';
                    return 1;
                }
                    
        }
        return 0;
    }
};