The fifth day of June training - double pointer

Preface

The content of today's algorithm is ： Double pointer

One 、 Reverse word prefix

         I'll give you a subscript from 0 Starting string word And a character ch . find ch First occurrence of subscript i , reverse word From the subscript 0 Start 、 Until the subscript i end （ Including subscript i ） That part of the character . If word There are no characters in ch , Then no operation is required .
for example , If word = “abcdefd” And ch = “d” , Then you should reverse From the subscript 0 Start 、 Until the subscript 3 end （ Including subscript 3 ）. The result string will be “dcbaefd” .
return Result string .

One 、 Ideas :

         Double pointer （1） for Under the cycle Again Define a pointer to move , If meet ch, Proceed from i To j Inter character exchange , And then end the loop ;
        （2） Can't find ch It ends the cycle ;

Two 、 Source code

class Solution {
    
public:
    string reversePrefix(string word, char ch) {
    
        for(int i=0;i<word.size();i++){
    
            int j=i;
            while(j<word.size()&&word[j]!=ch) j++;
            if(word[j]==ch){
    
                while(i<j){
    
                    int t=word[i];
                    word[i]=word[j];
                    word[j]=t;
                    ++i,--j;
                }
            }
            break;
        }
        return  word;
    }
};

3、 ... and . Knowledge point

         Application of double pointer , The exchange of elements in an array

Two 、 Just reverse the letters

         Give you a string s , Reverse the string according to the following rules ：
All non English letters remain in their original positions .
All English letters （ Lowercase or uppercase ） Position reversal .
Returns the inverted s .

One 、 Ideas :

         Define two pointers , Move from left to right if not Letters move , Scan right to left ; If it is not a letter Move , Then proceed Back and forth exchange , Loop until the pointer collides ;

Two 、 Source code

class Solution {
    
    bool isWord(char a){
    
        return a>='a'&&a<='z'||a>='A'&&a<='Z';
    }
    void swap(char *a,char *b){
    
        char t=*a;
        *a=*b;
        *b=t;
    }
public:
    string reverseOnlyLetters(string s) {
    
        int i=0,j=s.size()-1;
        while(i<j){
    
            if(!isWord(s[i])) {
    
                ++i;
                continue;
            }
            if(!isWord(s[j])) {
    
                --j;
                continue;
            }
            swap(&s[i],&s[j]);
            ++i,--j;
        }
        return s;
    }
};

3、 ... and . Knowledge point

         Application of double pointer