面试题4：字符串

解法1：从前往后扫描，碰到空格就替换，空格后面的元素向后移2。显然这种方式中有很多冗余的移动操作，其复杂度为O（n²)

解法2：先遍历一遍获得空格的总个数。 

#include<iostream>

using std::cout;
using std::cin;
using std::endl;

void ReplaceBlank(char str[],int length)
{
    if (str==NULL||length<=0)
    {
        std::cerr << "string is empty OR null" << endl;
        return;
    }

    int originlength = 0;
    int numberOfblank = 0;
    int i = 0;
    while (str[i]!='')
    {
        originlength++;
        
        if (str[i++] = ' ')
        {
            numberOfblank++;
        }
    }

    int newlength = originlength + 2 * numberOfblank;
    if (newlength>length)
    {
        return;
    }

    int newpos = newlength;
    int oripos = originlength;
    while (oripos>=0&&newpos>oripos)
    {
        if (str[oripos]==' ')
        {
            str[newpos--] = '0';
            str[newpos--] = '2';
            str[newpos--] = '%';
        }
        else
        {
            str[newpos--] = str[oripos];
        }

        --oripos;
    }
}

void Test(char* testName,char *str,int length,char *expected)
{
    if (testName!=NULL)
    {
        cout << testName << endl;
    }

    ReplaceBlank(str, length);

    if (str==NULL&&expected==NULL)
    {
        cout << "passed" << endl;
    }
    else if (strcmp(str,expected)==0)
    {
        cout << "passed" << endl;
    }
    else if (str!=NULL&&expected==NULL)
    {
        cout << "failed" << endl;
    }
    else
    {
        cout << "failed" << endl;
    }
}

//空格位于字符串最前面
void Test1()
{
    const int length = 100;

    char str[length] = " hello world";
    char *expected = "%20hello%20world";
    Test("test1", str, length, expected);
}

//空格位于字符串最后面
void Test2()
{
    const int length = 100;

    char str[length] = "hello world ";
    char *expected = "hello%20world%20";
    Test("test2", str, length, expected);
}

//有连续多个空格
void Test3()
{
    const int length = 100;

    char str[length] = "hello  world";
    char *expected = "hello%20%20world";
    Test("test3", str, length, expected);
}

//没有空格
void Test4()
{
    const int length = 100;

    char str[length] = "helloworld";
    char *expected = "helloworld";
    Test("test4", str, length, expected);
}

//字符串为NULL
void Test5()
{
    Test("test5", NULL, 0, NULL);
}

//空字符串
void Test6()
{
    const int length = 100;

    char str[length] = "";
    char *expected = "";
    Test("test6", str, length, expected);
}

//全是空格
void Test7()
{
    const int length = 100;

    char str[length] = "   ";
    char *expected = "%20%20%20";
    Test("test7", str, length, expected);
}
int main()
{
    Test1();
    Test2();
    Test3();
    Test4();
    Test5();
    Test6();
    Test7();

    return 0;
}