[04] Alternatively char character string, double pointer, from the back

topic

Please implement a function, a string to replace each space to "20%." For example, when the string is We Are Happy. After the string is replaced after We% 20Are% 20Happy.

Thinking

Since this problem is to char pointer parameter can not be used directly replace method of string, but others reference solution to a problem, found a more clever two-pointer, the method does not create additional string.

reward

Double pointer, the pointer points to the end of the original series 1, 2 just after the pointer at the end of the original string expansion, then decreasing jointly, 1 == pointer when the pointer is 2, indicating that the replacement is successful, the case only because there is no space, double pointer to overlap.

Code

class Solution {
public:
    void replaceSpace(char *str,int length) {
        if(str==NULL) return ;
        int CountOfSpace = 0;
        int OriginSize = 0;
        for(int i = 0;str[i]!='\0';i++){
            OriginSize++;
            if(str[i]==' ') 
                CountOfSpace++;
        }
        int len = OriginSize+2*CountOfSpace;
        if(len+1>length) return ;
        char *pstr1 = str+OriginSize;//结束符
        char *pstr2 = str+len;
        while(pstr2>pstr1){
            if(*pstr1==' '){
                *pstr2-- = '0';
                *pstr2-- = '2';
                *pstr2-- = '%';
            }
            else *pstr2--=*pstr1;
            pstr1--;
        }
    }
};