本文实例讲述了C++实现将一个字符串中的字符替换成另一个字符串的方法，分享给大家供大家参考。具体方法如下：
题目要求：
原地实现字符串中的每个空格替换成"%20"，例如输入"We are happy"， 输出"We%20are%20happy"
被替换的字符串当然不仅仅是空格，上面只是个例子
这是道很好的题目，也是百度面试中的一道题，题目不难，但是问题得考虑全面。这里给出如下实现代码：
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#include <iostream>
#include <vector>
#include <cassert>
using namespace std;
int findNumberFirst(const char *str, const char *dest, vector<int>& pvec)
{
if (str == NULL || dest == NULL)
return 0;
int pos = 0;
int lenStr = strlen(str);
int lenDest = strlen(dest);
if (lenStr < lenDest)
return 0;
int count = 0;
while (pos <= lenStr - lenDest)
{
if (strncmp(str + pos, dest, strlen(dest)) == 0)
{
pvec.push_back(pos);
pos += lenDest;
count++;
}
else
{
pos++;
}
}
return count;
}
int findNumberLast(const char *str, const char *dest, vector<int> &pvec)
{
if (str == NULL || dest == NULL)
return 0;
int strLen = strlen(str);
int destLen = strlen(dest);
if (strLen < destLen)
return 0;
int pos = 0;
while (pos <= strLen - destLen)
{
if (strncmp(str + pos, dest, strlen(dest)) == 0)
{
pos += destLen;
pvec.push_back(pos - 1);
}
else
{
pos++;
}
}
return pvec.size();
}
void replaceArray(char *str, const char *src, const char *dest)
{
if (str == NULL || src == NULL || dest == NULL)
return;
vector<int> pvec;
int strLen = strlen(str);
int srcLen = strlen(src);
int destLen = strlen(dest);
if (strLen < srcLen)
return;
int posBefore = 0;
int posAfter = 0;
if (srcLen < destLen)
{
int count = findNumberLast(str, src, pvec);
if (count <= 0)
return;
posAfter = strLen + count * (destLen - srcLen) - 1;
posBefore = strLen - 1;
while (count > 0 && posBefore >= 0)
{
if (pvec[count - 1] == posBefore)
{
posAfter -= destLen;
strncpy(str + posAfter + 1, dest, strlen(dest));
count--;
posBefore--;
}
else
{
str[posAfter--] = str[posBefore--];
}
}
}
else if (strLen > destLen)
{
int count = findNumberFirst(str, src, pvec);
if (count <= 0)
return;
posAfter = 0;
posBefore = 0;
int i = 0;
while (count >= 0 && posBefore < strLen)
{
if (count > 0 && pvec[i] == posBefore)
{
strncpy(str + posAfter, dest, strlen(dest));
posAfter += destLen;
count--;
posBefore += srcLen;
i++;
}
else
{
str[posAfter++] = str[posBefore++];
}
}
str[posAfter] = '\0';
}
}
void main()
{
char *str = new char[100];
if (str == NULL)
return;
memset(str, '\0', 100);
const char *src = " ";
const char *dest = "%20";
//case1: 只有1个空格
strcpy(str, " ");
replaceArray(str, src, dest);
cout << "str: " << str << endl;
replaceArray(str, dest, src);
cout << "str: " << str << endl;
//case2: 两个空格
strcpy(str, " ");
replaceArray(str, src, dest);
cout << "str: " << str << endl;
replaceArray(str, dest, src);
cout << "str: " << str << endl;
//case3: 正常情况
strcpy(str, "we are happy");
replaceArray(str, src, dest);
cout << "str: " << str << endl;
replaceArray(str, dest, src);
cout << "str: " << str << endl;
//case3: 空格在前
strcpy(str, " we are happy");
replaceArray(str, src, dest);
cout << "str: " << str << endl;
replaceArray(str, dest, src);
cout << "str: " << str << endl;
//case4: 空格在后
strcpy(str, "we are happy ");
replaceArray(str, src, dest);
cout << "str: " << str << endl;
replaceArray(str, dest, src);
cout << "str: " << str << endl;
//case4: 没空格
strcpy(str, "wearehappy");
replaceArray(str, src, dest);
cout << "str: " << str << endl;
replaceArray(str, dest, src);
cout << "str: " << str << endl;
//case5: 两边一样
strcpy(str, "we are happy");
replaceArray(str, src, dest);
cout << "str: " << str << endl;
src = "%20";
assert(dest == "%20");
replaceArray(str, dest, src);
cout << "str: " << str << endl;
}
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分析上述代码，很有意思的一个情况是srcLen和destLen或大或小的情形，其边界条件的判定不一样。比如we are happy为例子，从后往前拷贝时，count=2。
在count=0时，恰好将最前面的空格替换完成，we则不用重复拷贝。但是对于从前往后拷贝，当count=0时，最后面的happy将不会被拷贝。
希望本文所述实例对大家C++程序算法设计的学习有所帮助。
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