2021 CCPC Harbin E. power and modulo (thinking questions)

Problem - E - Codeforces

Ideas ：

This question makes clear what kinds of situations , Let's judge the first number , It should be noted that it cannot be used simply and directly according to the meaning of the question

pow（2,n - 1）, This will wa, because n yes 1e5 The order of magnitude . Here is a rule , namely 2^(n - 1) The law of the sequence of is   The previous one * 2 =  The latter , be 2^(n - 1) mod M after , The law has not changed , Is still   The previous one * 2 =  The latter

Code ：

#include<iostream>
#include<cstring>
#include<vector>
#include<string>
#include<algorithm>
#include<cmath>

using namespace std;

typedef long long ll;
int tmp[100010];
int x = 1;
int t;
bool flag, flag1;


int main() {
    scanf("%d", &t);

    int n;
    while(t --) {
        scanf("%d", &n);
        flag = false, flag1 = false, x = 1;
        for(int i = 1; i <= n; i ++) 
            scanf("%d", &tmp[i]);

        if(tmp[1] == 0) {
            for(int i = 1; i <= n; i ++) {
                if(tmp[i] != 0) {
                    flag = true;
                    break;
                }
            }
        }
        else if(tmp[1] > 1) flag = true;
        else {
             for(int i = 2; i <= n; i ++) {
            if(tmp[i - 1] * 2 != tmp[i]) {
                x = tmp[i - 1] * 2 - tmp[i];
                flag1 = true;
                break;
            }
        }
        if(flag1) {
            for(int i = 2; i <= n; i ++) {
            if(tmp[i - 1] * 2 % x != tmp[i]) {
                flag = true;
                break;
            }
        }
        } else 
        flag = true;
        } 
        if(!flag) printf("%d\n", x);
        else  printf("-1\n");
    }

    return 0;
}