Codeforces 311(div 2):E. Ann and Half-Palindrome

#include<bits\stdc++.h>
#define MAX_ASCII 2
using namespace std;
const int N = 5e+3 + 7;
bool dp[N][N];
char str[N];
int dict[N * N][MAX_ASCII + 1], cnt = 1;
//字典树插入
void Insert(const char *s, int L, int R)
{
    int v = 0, i = L;
    while (L <= R)
    {
        if (dict[v][s[L] - 'a'])
            v = dict[v][s[L++] - 'a'];
        else
            v = dict[v][s[L++] - 'a'] = cnt++;
        if (dp[i][L - 1])
            ++dict[v][2];
    }
}
//先序遍历查询第K大值
bool Rank(int v, vector<char> &path, int &k)
{
    k -= dict[v][2];
    if (k <= 0)
    {
        for (auto x : path)
            cout << x;
        cout << endl;
        return true;
    }
    for (int i = 0; i < 2; ++i)
    {
        if (dict[v][i])
        {
            path.push_back((char)(i + 'a'));
            if (Rank(dict[v][i], path, k))
                return true;
            path.pop_back();
        }
    }
    return false;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(NULL);
    int k, len;
    cin >> str >> k;
    len = strlen(str);
    for (int i = len - 1; i >= 0; --i)
        for (int j = len - 1; j >= i; --j)
            dp[i][j] = i <= j - 4 ? (str[i] == str[j] && dp[i + 2][j - 2]) : str[i] == str[j];
    for (int i = 0; i < len; ++i)
    {
        int j = len - 1;
        for (; j >= i && !dp[i][j]; --j);
        Insert(str, i, j);
    }
    vector<char> path;
    Rank(0, path, k);
    return 0;
}

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