HDU-2665-Kth number

题目传送门

sol：主席树，模板题

PS：这题从第一次错误提交到成功AC隔了半年，然后从半懂到现在会用了又隔了将近一年。前几天回顾了主席树，补上代码。

• 主席树

  #include "cstdio"
  #include "algorithm"
  using namespace std;
  const int MAXN = 1e5 + 5;
  struct Node {
      int lson, rson;
      int cnt;
  } tree[MAXN * 30];
  int root[MAXN], tot;
  int a[MAXN], b[MAXN];
  int build(int l, int r) {
      int i = ++tot;
      tree[i].cnt = 0;
      if (l != r) {
          int mid = l + r >> 1;
          tree[i].lson = build(l, mid);
          tree[i].rson = build(mid + 1, r);
      }
      return i;
  }
  int add(int pre, int l, int r, int index) {
      int i = ++tot;
      tree[i] = tree[pre];
      tree[i].cnt++;
      if (l != r) {
          int mid = l + r >> 1;
          if (index <= mid) tree[i].lson = add(tree[pre].lson, l, mid, index);
          else tree[i].rson = add(tree[pre].rson, mid + 1, r, index);
      }
      return i;
  }
  int query(int ql, int qr, int l, int r, int index) {
      if (l == r) return l;
      int sum = tree[tree[qr].lson].cnt - tree[tree[ql].lson].cnt;
      int mid = l + r >> 1;
      if (sum >= index) return query(tree[ql].lson, tree[qr].lson, l, mid, index);
      else return query(tree[ql].rson, tree[qr].rson, mid + 1, r, index - sum);
  }
  int main() {
      int t, n, m, q;
      int l, r, index;
      scanf("%d", &t);
      while (t--) {
          scanf("%d%d", &n, &q);
          tot = 0;
          for (int i = 1; i <= n; i++) {
              scanf("%d", &a[i]);
              b[i] = a[i];
          }
          sort(b + 1, b + 1 + n);
          m = unique(b + 1, b + 1 + n) - b - 1;
          root[0] = build(1, m);
          for (int i = 1; i <= n; i++) {
              index = lower_bound(b + 1, b + 1 + m, a[i]) - b;
              root[i] = add(root[i - 1], 1, m, index);
          }
          while (q--) {
              scanf("%d%d%d", &l, &r, &index);
              printf("%d\n", b[query(root[l - 1], root[r], 1, m, index)]);
          }
      }
      return 0;
  }