151. Reverse words in string - double pointer method

class Solution {
    
    public String reverseWords(String s) {
    
        //1.Remove the middle of the fore and aft, and extra Spaces
        StringBuilder sb = removeSpace(s);
        //2.反转整个字符串
        reverseString(sb,0,sb.length()-1);
        //sb.reverse();
        //3.Hair turn each word
        reverseEachWord(sb);

        return sb.toString();

    }
    private void reverseEachWord(StringBuilder sb){
    
        int start = 0;
        int end = 1;
        int n = sb.length();
        while(start<n){
    
            while(end<n && sb.charAt(end)!=' '){
    
                end++;
            }
            reverseString(sb,start,end-1);
            start=end+1;
            end=start+1;
        }
    }
    private void reverseString(StringBuilder sb,int start,int end){
    
        //可以使用reverse() 函数进行反转
        while(start<end){
    
            char c = sb.charAt(start);
            sb.setCharAt(start,sb.charAt(end)); //sb.setCharAt(int pos,char c) 位置 字符
            sb.setCharAt(end,c);
            //Or exchange method;reverse() 函数进行反转
            start++;
            end--;
        }
    }
    private StringBuilder removeSpace(String s){
    
        int start=0;
        int end = s.length()-1;
        while(s.charAt(start) ==' ') start++;
        while(s.charAt(end) ==' ') end--;
        StringBuilder sb = new StringBuilder();
        while(start<=end){
    
            char c = s.charAt(start);
            if(c!=' ' || sb.charAt(sb.length()-1)!=' '){
    
                sb.append(c);
            }
            start++;
        }
        return sb;
    }

}