leetcode 5088 Missing Number In Arithmetic Progression

https://leetcode-cn.com/problems/missing-number-in-arithmetic-progression/

算出等差数列的和。然后减去所有元素后剩下的就是少了的那个数字。

class Solution {
public:
    int missingNumber(vector<int>& arr) {
        int n = arr.size();
        int diff = (arr.back() - arr[0]) / n;
        int sum = 0;
        int first = arr[0];
        for(int i = 0; i <= n; i++)
        {
            sum += first;
            first += diff;
        }
        int sum2 = 0;
        for(int i : arr)
            sum2 += i;
        return sum - sum2;
    }
};