A1104. Sum of Number Segments
Given a sequence of positive numbers, a segment is defined to be a consecutive subsequence. For example, given the sequence {0.1, 0.2, 0.3, 0.4}, we have 10 segments: (0.1) (0.1, 0.2) (0.1, 0.2, 0.3) (0.1, 0.2, 0.3, 0.4) (0.2) (0.2, 0.3) (0.2, 0.3, 0.4) (0.3) (0.3, 0.4) (0.4).
Now given a sequence, you are supposed to find the sum of all the numbers in all the segments. For the previous example, the sum of all the 10 segments is 0.1 + 0.3 + 0.6 + 1.0 + 0.2 + 0.5 + 0.9 + 0.3 + 0.7 + 0.4 = 5.0.
Input Specification:
Each input file contains one test case. For each case, the first line gives a positive integer N, the size of the sequence which is no more than 105. The next line contains N positive numbers in the sequence, each no more than 1.0, separated by a space.
Output Specification:
For each test case, print in one line the sum of all the numbers in all the segments, accurate up to 2 decimal places.
Sample Input:
4 0.1 0.2 0.3 0.4
Sample Output:
5.00
#include<cstdio> #include<iostream> #include<algorithm> using namespace std; int main(){ int N; double ans = 0, temp; scanf("%d", &N); for(int i = 1; i <= N; i++){ scanf("%lf", &temp); ans += 1.0 * i * (N + 1 - i) * temp; } printf("%.2lf", ans); cin >> N; return 0; }
总结:
1、主要是找规律, 计算出每个数一共出现几次(与位置有关),然后做乘法再累加即可。第i个数(从1开始)出现的次数是:i * (n + 1 - i)。
2、i * (N + 1 - i)有可能超过int的范围。 所以做类型转换时,1.0应该乘在式子的最前面。否则当int溢出后再做转换就无效了。