Solve problems 18. Concatenated String from CS Academy

Author: AliOsm

CS Academy/18. Concatenated String.cpp

@@ -0,0 +1,55 @@
+/*
+  Idea:
+    - Build next array for string `a`, then using it iterate over string `b`
+      and each time try to find the current character `b[i]`, if you cannot
+      find it then return to the beginning of the string `a` and try to find
+      it again.
+*/
+
+#include <bits/stdc++.h>
+
+using namespace std;
+
+int const N = 1e5 + 1;
+int k = 1, nxt[N][26];
+bool vis[26];
+string a, b;
+
+int main() {
+  cin >> a >> b;
+  
+  for(int i = 0; i < a.length(); ++i)
+    vis[a[i] - 'a'] = true;
+  for(int i = 0; i < b.length(); ++i)
+    if(!vis[b[i] - 'a']) {
+      cout << -1 << endl;
+      return 0;
+    }
+  
+  memset(nxt, -1, sizeof nxt);
+  for(int i = a.length() - 2; i >= 0; --i) {
+    for(int j = 0; j < 26; ++j)
+      nxt[i][j] = nxt[i + 1][j];
+    nxt[i][a[i + 1] - 'a'] = i + 1;
+  }
+  
+  for(int i = 0, whr = -1; i < b.length(); ++i) {
+    char cur = b[i];
+    
+    if(whr == -1) {
+      if(a[0] == cur)
+        whr = 0;
+      else
+        whr = nxt[0][cur - 'a'];
+    } else {
+      if(nxt[whr][cur - 'a'] != -1)
+        whr = nxt[whr][cur - 'a'];
+      else
+        whr = -1, ++k, --i;
+    }
+  }
+  
+  cout << k << endl;
+  
+  return 0;
+}

CS Academy/README.md

@@ -1,5 +1,6 @@
 # Problems links
 
+- [18. Concatenated String](https://csacademy.com/contest/round-18/task/concatenated-string/)
 - [18. Consecutive Digits Signs](https://csacademy.com/contest/round-18/task/consecutive-digit-signs/)
 - [24. Vector Size](https://csacademy.com/contest/round-24/#task/vector-size)
 - [24. Kth Special Number](https://csacademy.com/contest/round-24/#task/kth-special-number)