Substring Score——The 11th Blue Bridge Cup AC

Substring Score——The 11th Blue Bridge Cup AC

topic description

original address

For a string S, we define the score f ( S ) of S as the number of characters in S that appear exactly once.


For example f("aba")=1, f("abc")=3
f("aaa")=0.


Now given a string S [ 0… n − 1 ], please calculate the non-empty substring S [ i . . j ] S[i…j]S[i…j] for all S, f ( S [ i . . j ] )
What is the sum of f(S[i…j])f(S[i…j]) .




Input format:
Enter a line containing a string S composed of lowercase letters.


Output Format
Output an integer representing the answer.


input sample
ababc


output sample
21

train of thought

• We require the sum of all non-empty substrings with one occurrence, that is, the sum of all strings that occur once based on each element of the string;
• Use pre[i] to record the position where the i-th letter appeared last time, then it can extend to the left up to pre[i] + 1, and there are i - pre[i] letters in total to the i-th letter;
• Use next[i] to record the position where the i-th letter appears next time, similarly, it can extend to the right up to next[i] - 1, and there are next[i] - i letters to the i-th letter;
• The multiplication of the two is the number of all substrings in which the letter appears once;

the code

#include <iostream>
// memset 需要 cstring 头文件
#include <cstring>
using namespace std;
typedef long long ll;
const int N = 1e5 + 10;
int pre[N], nex[N];
int idx[26];
int main() {
    
    
    string s;
    cin >> s;
    s = " " + s;
    ll res = 0;
    int n = s.size();
    for (int i = 1; i < n; ++i) {
    
    
        pre[i] = idx[s[i] - 'a'];
        idx[s[i] - 'a'] = i;
    }
    for (int i = 0; i < 26; ++i)
        idx[i] = n;
    for (int i = n - 1; i; --i) {
    
    
        nex[i] = idx[s[i] - 'a'];
        idx[s[i] - 'a'] = i;
    }
    for (int i = 1; i < n; ++i) {
    
    
        res += (ll) (i - pre[i]) * (nex[i] - i);
    }
    cout << res;
    return 0;
}

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[Blue Bridge Cup 2020 Preliminary Competition] substring score and problem solution C++