PAT甲级 1173 How Many Ways to Buy a Piece of Land（25）

方法：前缀和+二分查找

类似题目：力扣 300. 最长递增子序列

#include <bits/stdc++.h>
using namespace std;
int M, N;
int lands[10005];

int binarySearch(int *prefixSum, int i){
	int left = 0, right = i - 1;
	while (left <= right) {
		int mid = left + ((right - left) >> 1);
		if (prefixSum[i] - prefixSum[mid] < M) {
			right = mid - 1;
		} else if (prefixSum[i] - prefixSum[mid] > M) {
			left = mid + 1;
		} else {
			return i - mid;
		}
	}
	return i - left;
}

int main() {
	cin >> N >> M;
	for (int i = 0; i < N; ++i) {
		cin >> lands[i];
	}
	int *prefixSum = new int[N + 1];
	prefixSum[0] = 0;
	int sum = 0;
	for (int i = 1; i <= N; ++i) {
		prefixSum[i] = prefixSum[i - 1] + lands[i - 1];
		sum += binarySearch(prefixSum, i);
	}
	cout << sum << endl;
}