区间第k大 可修改主席树，树状数组套线段树。

#include <cstdio>
#include <vector>
using namespace std;

const int N = 100010;

struct Node {
int cnt;
Node *ls, *rs;
}pool[N * 200], *tail = pool, *roots
, *null;

int n, q;
int aa
;
Node *va[10000], *vb[10000];
int ca, cb;

void init() {
null = ++tail;
null->ls = null;
null->rs = null;
null->cnt = 0;
}
Node *newnode() {
Node *nd = ++tail;
nd->ls = null;
nd->rs = null;
nd->cnt = 0;
return nd;
}
void modify( int lf, int rg, int pos, int delta ) {
for( int t = 0; t < ca; t++ )
va[t]->cnt += delta;
if( lf == rg )
return;
int mid = (lf + rg)>>1;
if( pos <= mid ) {
for( int t = 0; t < ca; t++ ) {
if( va[t]->ls == null )
va[t]->ls = newnode();
va[t] = va[t]->ls;
}
modify( lf, mid, pos, delta );
} else {
for( int t = 0; t < ca; t++ ) {
if( va[t]->rs == null )
va[t]->rs = newnode();
va[t] = va[t]->rs;
}
modify( mid+1, rg, pos, delta );
}
}

int query_seg( int lf, int rg, int k ) {
if( lf == rg ) return lf;
int mid = (lf + rg) >> 1;
int lz = 0;
for( int t = 0; t < ca; t++ ) lz += va[t]->ls->cnt;
for( int t = 0; t < cb; t++ ) lz -= vb[t]->ls->cnt;
if( k <= lz ) {
for( int t = 0; t < ca; t++ ) va[t] = va[t]->ls;
for( int t = 0; t < cb; t++ ) vb[t] = vb[t]->ls;
return query_seg( lf, mid, k );
} else {
for( int t = 0; t < ca; t++ ) va[t] = va[t]->rs;
for( int t = 0; t < cb; t++ ) vb[t] = vb[t]->rs;
k -= lz;
return query_seg( mid+1, rg, k );
}
}
void modify( int u, int x, int delta ) {
ca = 0;
for( int i = u; i <= n; i += i & -i )
va[ca++] = roots[i];
modify( 1, n, x, delta );
}
void modify( int u, int x ) {
modify( u, aa[u], -1 );
modify( u, x, +1 );
aa[u] = x;
}
int query( int l, int r, int k ) {
ca = cb = 0;
for( int i = r; i; i -= i & -i )
va[ca++] = roots[i];
for( int i = l - 1; i; i -= i & -i )
vb[cb++] = roots[i];
return query_seg(1,n,k);
}

int main() {
scanf( "%d%d", &n, &q );
for( int i = 1; i <= n; i++ )
scanf( "%d", aa + i );
init();
for( int i = 1; i <= n; i++ )
roots[i] = newnode();
for( int i = 1; i <= n; i++ )
modify( i, aa[i], +1 );
while( q-- ) {
char ss[100];
scanf( "%s", ss );
if( ss[0] == 'q' ) {
int l, r, k;
scanf( "%d%d%d", &l, &r, &k );
printf( "%d\n", query(l,r,k) );
} else {
int pos, val;
scanf( "%d%d", &pos, &val );
modify( pos, val );
}
}
//  fprintf( stderr, "mem used %d%%\n", int(tail - pool) * 100 / int(sizeof(pool)/sizeof(pool[0])) );
}