uva 10029 HASH + DP

最新推荐文章于 2021-08-30 14:52:22 发布

AlmostFree

最新推荐文章于 2021-08-30 14:52:22 发布

阅读量359

点赞数

CC 4.0 BY-SA版权

分类专栏：动态规划

本文链接：https://blog.youkuaiyun.com/u013508213/article/details/44539893

动态规划专栏收录该内容

70 篇文章

订阅专栏

本文介绍了一种结合哈希与动态规划的方法来解决给定字典中单词间通过增删改操作转换的问题，旨在寻找转换序列中最长的单词。

题意：

给一个字典，里面有好多单词。单词可以由增加、删除、变换，变成另一个单词，问能变换的最长单词长度。

解析：

HASH+dp

代码：

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <stack>
#include <vector>
#include <queue>
#include <map>
#include <climits>
#include <cassert>
#define LL long long

using namespace std;
const int inf = 0x3f3f3f3f;
const double eps = 1e-8;
const double pi = 4 * atan(1.0);
const double ee = exp(1.0);

const int maxn = 25000 + 10;
const int HASH = 1000003;

int n;
int head[HASH], next[maxn];
char s[maxn][20], t[20];
int dp[maxn];

//hash ...
int Hash(char str[])
{
    int v = 0, seed = 88;
    while (*str)
        v = v * seed + *(str++);
    return (v & 0x7fffffff) % HASH;
}

void Insert(int x)
{
    int h = Hash(s[x]);
    next[x] = head[h];
    head[h] = x;
}

int Search()
{
    int i;
    int h = Hash(t);//
    for (i = head[h]; i != -1; i = next[i])
    {
        if (strcmp(t, s[i]) == 0)
        {
            break;
        }
    }
    return i;
}

//dp ...
void add(char str[], int pos, int key)
{
    memset(t, 0, sizeof(t));
    int i, j;
    for (i = 0, j = 0; i < pos; i++, j++)
    {
        t[j] = str[i];
    }
    t[j++] = 'a' + key;
    for (; str[i]; i++, j++)
    {
        t[j] = str[i];
    }
    t[j] = '\0';
}

void del(char str[], int pos)
{
    memset(t, 0, sizeof(t));
    int i, j;
    for (i = 0, j = 0; i < pos; i++, j++)
    {
        t[j] = str[i];
    }
    i++;
    for (; str[i]; i++, j++)
    {
        t[j] = str[i];
    }
    t[j] = '\0';
}

void change(char str[], int pos, int key)
{
    memset(t, 0, sizeof(t));
    strcpy(t, str);
    t[pos] = 'a' + key;
}

int Dfs(int cur)
{
    if (dp[cur] != -1)
        return dp[cur];
    int res = 0;
    int len = strlen(s[cur]);
    //add
    for (int i = 0; i <= len; i++)
    {
        for (int k = 0; k < 26; k++)
        {
            add(s[cur], i, k);
            int pos = Search();
            if (pos != -1 && strcmp(s[cur], t) < 0)
            {
                int t = Dfs(pos) + 1;///pos
                if (res < t)
                {
                    res = t;
                }
            }
        }
    }
    //delete
    for (int i = 0; i < len; i++)
    {
        del(s[cur], i);
        int pos = Search();
        if (pos != -1 && strcmp(s[cur], t) < 0)
        {
            int t = Dfs(pos) + 1;///pos
            if (res < t)
            {
                res = t;
            }
        }
    }
    //change
    for (int i = 0; i < len; i++)
    {
        for (int k = 0; k < 26; k++)
        {
            change(s[cur], i, k);
            int pos = Search();
            if (pos != -1 && strcmp(s[cur], t) < 0)
            {
                int t = Dfs(pos) + 1;///pos
                if (res < t)
                {
                    res = t;
                }
            }
        }
    }
    return dp[cur] = res;
}

int main()
{
#ifdef LOCAL
    freopen("in.txt", "r", stdin);
#endif // LOCAL
    n = 0;
    memset(head, -1, sizeof(head));
    while (scanf("%s", s[n]) != EOF)
    {
        Insert(n);
        n++;
    }
    memset(dp, -1, sizeof(dp));
    int ans = 0;
    for (int i = 0; i < n; i++)
    {
        int t = Dfs(i) + 1;
        if (ans < t)
        {
            ans = t;
        }
    }
    printf("%d\n", ans);
    return 0;
}