leetcode 28

题目描述：

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

分析：

这个题目考察的是KMP算法，这个算法的核心是求出子串的自匹配数组，在失配情况下不回溯父串指针（其实是索引），只需将子串向左滑动即可，next数组就是记录了滑动的距离的数组，求next数组是这个算法的难点。

我的第一个版本的代码如下：

#include <iostream>
#include <string>
#include <vector>

using namespace std;

int strStr(string haystack, string needle);
void getNext(vector<int> &ivec, const string & s);

int main()
{
    cout << "Hello world!" << endl;
    string hay("mississippi");
    string needle("issi");
    int res = strStr(hay, needle);
    cout << "the result of matching:" << res << endl;
    system("pause");
    return 0;
}

void getNext(vector<int> &ivec, const string & s)
{
    int i = 0, j = -1;
    ivec[i] = j;
    while (i < s.size() - 1)
    {
        if (j == -1 || s[i] == s[j])
            ivec[++i] = ++j;
        else
            j = ivec[j];
    }
}

int strStr(string haystack, string needle)
{
    if (needle.size() == 0)
        return 0;
    if (haystack.size() == 0)
        return -1;

    vector<int> ivec(needle.size(), 0);
    getNext(ivec, needle);


    for (auto i = ivec.begin(); i != ivec.end(); i++)
        cout << *i << endl;


    int i = 0, j = 0;
    cout << haystack.size() << "  " << needle.size() << endl;

    while( i < haystack.size() && j < needle.size() )
    //while (i != haystack.size() && j != needle.size())
    {
        cout << "before modified " << i << " " << j << endl;
        if (j == -1 || haystack[i] == needle[j])
        {
            cout << "what" << endl;
            ++i;
            ++j;
        }
        else
        {
            j = ivec[j];
        }
        cout << "after modified " << i << " " << j << endl;

    }
    if (j == needle.size())
        return i - j;
    else
        return -1;

}

这份代码我以为可以完美AC了，结果报了wrong answer，我很不理解，经过修改，就是将上一个版本中的while循环头换成下方紧邻的那一行就AC了，经过和大神的讨论，大神很快指出了我的错误所在，那就是C++的隐式类型转换！haystack.size()返回的是一个unsigned int类型，而i， j是int型，在运算时int会隐式转换为unsigned int，而-1转换为unsigned int型之后为4294967295（2的32次方减1）是一个非常大的数字！以前写循环不注意，现在吃了大亏！希望吃一堑长一智吧，以后写代码一定要非常注意类型的选择，注意int和unsigned int不能比较，如果必须比较一定将unsigned int 强制转换为long long类型再比较！最后完成版的代码如下：

#include <iostream>
#include <string>
#include <vector>

using namespace std;

int strStr(string haystack, string needle);
void getNext(vector<int> &ivec, const string & s);

int main()
{
    string hay("mississippi");
    string needle("issi");
    int res = strStr(hay, needle);
    cout << "the result of matching:" << res << endl;
    system("pause");
    return 0;
}

void getNext(vector<int> &ivec, const string & s)
{
    int i = 0, j = -1;
    ivec[i] = j;
    while (i < s.size() - 1)
    {
        if (j == -1 || s[i] == s[j])
            ivec[++i] = ++j;
        else
            j = ivec[j];
    }
}

int strStr(string haystack, string needle)
{
    if (needle.size() == 0)
        return 0;
    if (haystack.size() == 0)
        return -1;

    vector<int> ivec(needle.size(), 0);
    getNext(ivec, needle);


    for (auto i = ivec.begin(); i != ivec.end(); i++)
        cout << *i << endl;


    int i = 0, j = 0;
    long long haystack_size = haystack.size();
    long long needle_size = needle.size();

    while( i < haystack_size && j < needle_size )
    {
        cout << "i and j before modified " << i << " " << j << endl;
        if (j == -1 || haystack[i] == needle[j])
        {
            cout << "what" << endl;
            ++i;
            ++j;
        }
        else
        {
            j = ivec[j];
        }
        cout << "i and j after modified " << i << " " << j << endl;
        haystack_size = haystack.size();
        needle_size = needle.size();

    }
    if (j == needle.size())
        return i - j;
    else
        return -1;

}