POJ.3415 Common Substrings

1.Problem
3415 -- Common Substrings (poj.org)

2.Idea
Suffix Array
Monotonic Stack

3.Source

 const int MAX_L = 100000 + 1;  
 const int MAX_N = 2 * MAX_L + 1;  
 std::string S;  
 int n, k;  
 int sa[MAX_N + 1], lcp[MAX_N + 1];   
 int Rank[MAX_N + 1], tmp[MAX_N + 1];  
 bool compare_sa(const int i, const int j)  
 {  
      if (Rank[i] != Rank[j])  
      {  
           return Rank[i] < Rank[j];  
      }  
      return (i + k <= n ? Rank[i + k] : -1) < (j + k <= n ? Rank[j + k] : -1);  
 }  
 void construct_sa()  
 {  
      for (int i = 0; i <= n; i++)  
      {  
           sa[i] = i;  
           Rank[i] = i < n ? S[i] : -1;  
      }  
      for (k = 1; k <= n; k <<= 1)  
      {  
           std::sort(sa, sa + n + 1, compare_sa);  
           tmp[sa[0]] = 0;  
           for (int i = 1; i <= n; i++)  
           {  
                tmp[sa[i]] = tmp[sa[i - 1]] + (compare_sa(sa[i - 1], sa[i]) ? 1 : 0);  
           }  
           for (int i = 0; i <= n; i++)  
           {  
                Rank[i] = tmp[i];  
           }  
      }  
 }  
 void construct_lcp()  
 {  
      memset(lcp, 0, sizeof(lcp));  
      for (int i = 0; i <= n; i++)  
      {  
           Rank[sa[i]] = i;  
      }  
      int h = 0;  
      lcp[0] = 0;  
      for (int i = 0; i < n; i++)  
      {  
           int j = sa[Rank[i] - 1];  
           if (h > 0)  
           {  
                h--;  
           }  
           for (; i + h < n && j + h < n && S[i + h] == S[j + h]; h++);  
           lcp[Rank[i] - 1] = h;  
      }  
 }  
 int Stack[MAX_N][2];  
 long long contribution, top;  
 using namespace std;  
 long long calc(int K, int l1, bool is_s1)  
 {  
      long long ans = 0;  
      for (int i = 0; i < n; i++) {  
           if (lcp[i] < K) {  
                top = contribution = 0;  
           }  
           else {  
                int size = 0;  
                if ((is_s1 && sa[i] < l1) || (!is_s1 && sa[i] > l1)) {  
                     ++size;  
                     contribution += lcp[i] - K + 1;  
                }  
                while (top > 0 && lcp[i] <= Stack[top - 1][0]) {  
                     --top;  
                     contribution -= Stack[top][1] * (Stack[top][0] - lcp[i]);  
                     size += Stack[top][1];  
                }  
                if (size) {  
                     Stack[top][0] = lcp[i];  
                     Stack[top][1] = size;  
                     ++top;  
                }  
                if ((is_s1 && sa[i + 1] > l1) || (!is_s1 && sa[i + 1] < l1)) {  
                     ans += contribution;  
                }  
           }  
      }  
      return ans;  
 }  
 void solve()  
 {  
      int k;  
      while (scanf("%d", &k), k) {  
           string s1, s2;  
           cin >> s1 >> s2;  
           int l1 = s1.length();  
           int l2 = s2.length();  
           S = s1 + "$" + s2;  
           n = l1 + l2 + 1;  
           construct_sa();  
           construct_lcp();  
           printf("%lld\n", calc(k, l1, true) + calc(k, l1, false));  
      }  
 }  
 int main() {  
      ios_base::sync_with_stdio(0); cin.tie(0); cout << fixed << setprecision(13);  
      solve();  
      return 0;  
 }