PalindromeTree.cpp

/**************************************************************************************

    Palindrome tree. Useful structure to deal with palindromes in strings. O(N)

    This code counts number of palindrome substrings of the string.
    Based on problem 1750 from informatics.mccme.ru:
    http://informatics.mccme.ru/moodle/mod/statements/view.php?chapterid=1750

**************************************************************************************/

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <string>
#include <utility>
#include <cstring>
#include <cassert>
#include <cmath>
#include <stack>
#include <queue>

using namespace std;

const int MAXN = 105000;

struct node {
    int next[26];
    int len;
    int sufflink;
    int num;
};

int len;
char s[MAXN];
node tree[MAXN]; 
int num;            // node 1 - root with len -1, node 2 - root with len 0
int suff;           // max suffix palindrome
long long ans;

bool addLetter(int pos) {
    int cur = suff, curlen = 0;
    int let = s[pos] - 'a';

    while (true) {
        curlen = tree[cur].len;
        if (pos - 1 - curlen >= 0 && s[pos - 1 - curlen] == s[pos])     
            break;  
        cur = tree[cur].sufflink;
    }       
    if (tree[cur].next[let]) {  
        suff = tree[cur].next[let];
        return false;
    }

    num++;
    suff = num;
    tree[num].len = tree[cur].len + 2;
    tree[cur].next[let] = num;

    if (tree[num].len == 1) {
        tree[num].sufflink = 2;
        tree[num].num = 1;
        return true;
    }

    while (true) {
        cur = tree[cur].sufflink;
        curlen = tree[cur].len;
        if (pos - 1 - curlen >= 0 && s[pos - 1 - curlen] == s[pos]) {
            tree[num].sufflink = tree[cur].next[let];
            break;
        }       
    }           

    tree[num].num = 1 + tree[tree[num].sufflink].num;

    return true;
}

void initTree() {
    num = 2; suff = 2;
    tree[1].len = -1; tree[1].sufflink = 1;
    tree[2].len = 0; tree[2].sufflink = 1;
}

int main() {
    //assert(freopen("input.txt", "r", stdin));
    //assert(freopen("output.txt", "w", stdout));

    gets(s);
    len = strlen(s);

    initTree();

    for (int i = 0; i < len; i++) {
        addLetter(i);
        ans += tree[suff].num;
    }

    cout << ans << endl;

    return 0;
}