题目大意
给你一个字符串，问substr(l,r)第k次出现的第一个字符的下标。
解题思路
对于所有满足条件的子串，以其首字母开头的所有后缀的lcp一定都是大于等于这个子串长度的，根据lcp的性质，(lcp(i, j) = min(lcp(k_1, k_2), i leq k_1,k_2 leq j)。我们可以得到如果将所有子串按字典序排序，则所有满足条件的后缀必定是一个连续区间，那么第k次出现的下标就是这个区间第k小的数。
具体写法是，我们可以先求出后缀数组，然后按排名将sa[i]插入主席树中，并且用st表预处理出height数组的区间最小值。对于每个询问，我们可以知道其所在的后缀的排名t，然后我们再用st表二分出其所在的lcp大于等于r-l+1的区间的左右端点L，R，最后用主席树求出区间第k小即可。
代码
const int maxn = 1e5+10;
char s[maxn];
int n, m;
int sa[maxn], x[maxn], y[maxn], c[maxn];
void get_sa() {
for (int i = 1; i<=n; ++i) ++c[x[i]=s[i]];
for (int i = 2; i<=m; ++i) c[i] += c[i-1];
for (int i = n; i; --i) sa[c[x[i]]--] = i;
for (int k = 1; k<=n; k<<=1) {
int num = 0;
for (int i = n-k+1; i<=n; ++i) y[++num] = i;
for (int i = 1; i<=n; ++i)
if (sa[i]>k) y[++num] = sa[i]-k;
for (int i = 1; i<=m; ++i) c[i] = 0;
for (int i = 1; i<=n; ++i) ++c[x[i]];
for (int i = 2; i<=m; ++i) c[i] += c[i-1];
for (int i = n; i; --i) sa[c[x[y[i]]]--] = y[i], y[i] = 0;
swap(x, y);
x[sa[1]] = 1, num = 1;
for (int i = 2; i<=n; ++i)
x[sa[i]] = (y[sa[i]]==y[sa[i-1]] && y[sa[i]+k]==y[sa[i-1]+k]) ? num:++num;
if (num==n) break;
m = num;
}
}
int rk[maxn], h[maxn];
void get_h() {
for (int i = 1; i<=n; ++i) rk[sa[i]] = i;
for (int i = 1, k = 0; i<=n; ++i) {
if (rk[i]==1) continue;
if (k) --k;
int j = sa[rk[i]-1];
while(i+k<=n && j+k<=n && s[i+k]==s[j+k]) ++k;
h[rk[i]] = k;
}
}
int f[maxn][20], lg[maxn];
void build() {
for (int i = n; i; --i) {
f[i][0] = h[i];
for (int j = 1; j<21; ++j)
if (i+(1<<j-1)<=n) f[i][j] = min(f[i][j-1], f[i+(1<<j-1)][j-1]);
}
for (int i = 2; i<=n; ++i) lg[i] = lg[i/2]+1;
}
int query(int l, int r) {
if (l>r) return INF;
int len = r-l+1;
int k = lg[len];
return min(f[l][k], f[r-(1<<k)+1][k]);
}
struct Node {
int l, r, sum;
} hjt[maxn*40];
int tot, rt[maxn];
void init() {
tot = 0;
clr(c, 0); clr(h, 0);
}
void insert(int pre, int &now, int l, int r, int pos) {
now = ++tot;
hjt[now] = hjt[pre];
++hjt[now].sum;
int mid = (l+r)>>1;
if (l==r) return;
if (pos<=mid) insert(hjt[pre].l, hjt[now].l, l, mid, pos);
else insert(hjt[pre].r, hjt[now].r, mid+1, r, pos);
}
int ask(int pre, int now, int l, int r, int k) {
if (l==r) return l;
int mid = (l+r)>>1;
int t = hjt[hjt[now].l].sum-hjt[hjt[pre].l].sum;
if (k<=t) return ask(hjt[pre].l, hjt[now].l, l, mid, k);
else return ask(hjt[pre].r, hjt[now].r, mid+1, r, k-t);
}
int main() {
IOS;
int __; cin >> __;
while(__--) {
init();
int q; cin >> n >> q;
cin >> s+1;
m = 122;
get_sa(); get_h(); build();
//for (int i = 1; i<=n; ++i) cout << h[i] << ' ' << rk[i] << endl;
for (int i = 1; i<=n; ++i) insert(rt[i-1], rt[i], 1, n, sa[i]);
while(q--) {
int ql, qr, k; cin >> ql >> qr >> k;
//cout << ql << endl;
int len = qr-ql+1;
int t = rk[ql];
int L = 1, R = t-1;
while(L<R) {
int mid = (L+R)>>1;
if (query(mid+1, t)>=len) R = mid;
else L = mid+1;
}
int l = L;
if (query(L+1, t)<len) ++l;
L = t+1, R = n;
while(L<R) {
int mid = (L+R+1)>>1;
if (query(t+1, mid)>=len) L = mid;
else R = mid-1;
}
int r = R;
if (query(t+1, r)<len) --r;
if (l>r && k==1) {
cout << ql << endl;
continue;
}
if (r-l+1<k) cout << -1 << endl;
else cout << ask(rt[l-1], rt[r], 1, n, k) << endl;
}
}
return 0;
}