【HDOJ】3068 最长回文

马拉车算法O(n)可解。

 1 /* 3068 */
 2 #include <iostream>
 3 #include <string>
 4 #include <map>
 5 #include <queue>
 6 #include <set>
 7 #include <stack>
 8 #include <vector>
 9 #include <deque>
10 #include <algorithm>
11 #include <cstdio>
12 #include <cmath>
13 #include <ctime>
14 #include <cstring>
15 #include <climits>
16 #include <cctype>
17 #include <cassert>
18 #include <functional>
19 using namespace std;
20 //#pragma comment(linker,"/STACK:102400000,1024000")
21
22 #define sti                set<int>
23 #define stpii            set<pair<int, int> >
24 #define mpii            map<int,int>
25 #define vi                vector<int>
26 #define pii                pair<int,int>
27 #define vpii            vector<pair<int,int> >
28 #define rep(i, a, n)     for (int i=a;i<n;++i)
29 #define per(i, a, n)     for (int i=n-1;i>=a;--i)
30 #define clr                clear
31 #define pb                 push_back
32 #define mp                 make_pair
33 #define fir                first
34 #define sec                second
35 #define all(x)             (x).begin(),(x).end()
36 #define SZ(x)             ((int)(x).size())
37 #define lson            l, mid, rt<<1
38 #define rson            mid+1, r, rt<<1|1
39
40 const int maxn = 110005;
41 char s[maxn*2], ss[maxn];
42 int P[maxn*2];
43 int ans, l;
44
45 void init() {
46     int i, len = strlen(ss);
47
48     l = 0;
49     s[l++] = ‘@‘;
50     s[l++] = ‘#‘;
51     for (i=0; i<len; ++i) {
52         s[l++] = ss[i];
53         s[l++] = ‘#‘;
54     }
55     s[l++] = ‘\0‘;
56 }
57
58 void manacher() {
59     int i, mx = 0, id;
60
61     for (i=1; i<l; ++i) {
62         P[i] = mx>i ? min(P[2*id-i], mx-i) : 1;
63         while (s[i+P[i]] == s[i-P[i]])
64             ++P[i];
65         if (i+P[i] > mx) {
66             mx = i + P[i];
67             id = i;
68         }
69     }
70
71     ans = 0;
72     for (i=1; i<l; ++i)
73         ans = max(ans, P[i]);
74     printf("%d\n", ans-1);
75 }
76
77 void solve() {
78     init();
79     manacher();
80 }
81
82 int main() {
83     ios::sync_with_stdio(false);
84     #ifndef ONLINE_JUDGE
85         freopen("data.in", "r", stdin);
86         freopen("data.out", "w", stdout);
87     #endif
88
89     while (scanf("%s", ss)!=EOF) {
90         solve();
91     }
92
93     #ifndef ONLINE_JUDGE
94         printf("time = %d.\n", (int)clock());
95     #endif
96
97     return 0;
98 }