[洛谷P3567][POI2014]KUR-Couriers

题目大意：给一个数列，每次询问一个区间内有没有一个数出现次数超过一半。有，输出这个数，否则输出$0$

题解：主席树，查询区间第$\bigg\lfloor\dfrac{len+1}{2}\bigg\rfloor$大，并判断它出现次数是否大于$\Big\lfloor\dfrac{len}{2}\Big\rfloor$

卡点：无

 

C++ Code：

#include <cstdio>
#define maxn 500010
#define N (maxn * 22)
int n, m;
int root[maxn];
int sz[N], lc[N], rc[N], idx;
void insert(int &rt, int l, int r, int num) {
	lc[++idx] = lc[rt], rc[idx] = rc[rt], sz[idx] = sz[rt] + 1, rt = idx;
	if (l == r) return ;
	int mid = l + r >> 1;
	if (num <= mid) insert(lc[rt], l, mid, num);
	else insert(rc[rt], mid + 1, r, num);
}
int query(int L, int R, int l, int r, int k, int len) {
	if (l == r) {
		if (sz[R] - sz[L] > len / 2) return l;
		return 0;
	}
	int mid = l + r >> 1, SZ = sz[lc[R]] - sz[lc[L]];
	if (k <= SZ) return query(lc[L], lc[R], l, mid, k, len);
	else return query(rc[L], rc[R], mid + 1, r, k - SZ, len);
}
 
int main() {
	scanf("%d%d", &n, &m);
	for (int i = 1, x; i <= n; i++) {
		scanf("%d", &x);
		root[i] = root[i - 1];
		insert(root[i], 1, n, x);
	}
	while (m --> 0) {
		int l, r;
		scanf("%d%d", &l, &r);
		printf("%d\n", query(root[l - 1], root[r], 1, n, r - l + 2 >> 1, r - l + 1));
	}
	return 0;
}