【BZOJ3524】[Poi2014]Couriers【主席树】

【题目链接】

比较裸的主席树，注意下标不要手滑写错了。

/* Pigonometry */
#include <cstdio>
#include <algorithm>

using namespace std;

const int maxn = 500005, maxnode = 10000005;

int n, m;

int son[maxnode][2], sum[maxnode], segcnt, root[maxn];

inline int iread() {
int f = 1, x = 0; char ch = getchar();
for(; ch < '0' || ch > '9'; ch = getchar()) f = ch == '-' ? -1 : 1;
for(; ch >= '0' && ch <= '9'; ch = getchar()) x = x * 10 + ch - '0';
return f * x;
}

inline void insert(int x, int &y, int l, int r, int c) {
y = ++segcnt;
sum[y] = sum[x] + 1;
if(l == r) return;
int mid = l + r >> 1;
son[y][0] = son[x][0]; son[y][1] = son[x][1];
if(c <= mid) insert(son[x][0], son[y][0], l, mid, c);
else insert(son[x][1], son[y][1], mid + 1, r, c);
}

inline int query(int x, int y, int k) {
int l = 1, r = n;
while(l < r) {
int mid = l + r >> 1;
if(sum[son[y][0]] - sum[son[x][0]] > k) x = son[x][0], y = son[y][0], r = mid;
else if(sum[son[y][1]] - sum[son[x][1]] > k) x = son[x][1], y = son[y][1], l = mid + 1;
else return 0;
}
return l;
}

int main() {
n = iread(); m = iread();
for(int i = 1; i <= n; i++) {
int x = iread();
insert(root[i - 1], root[i], 1, n, x);
}
while(m--) {
int l = iread(), r = iread();
printf("%d\n", query(root[l - 1], root[r], (r - l + 1) >> 1));
}
return 0;
}