557. Reverse Words in a String I

这题没啥好说的了

class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();
        int pos =0;
        int len =s.length();
        while(pos<len)
        {
            while(pos<len&&s.charAt(pos)==' ')
                {
                    sb.append(' ');
                    pos++;
            }
            if(pos==len) return sb.toString();
            int start = pos;
            while(pos<len&&s.charAt(pos)!=' ')
            {
                    pos++;
            }
            int end = pos-1;
            //  因为 pos的位置在第一个为空格的数。
            exchange(sb,s,start,end);
        }
        return sb.toString();
    }
    private void exchange (StringBuilder sb ,String s,int start ,int end)
    {
        for(int i = end;i>=start;i--)
        {
            sb.append(s.charAt(i));
        }
    }
}