算法1

用split将字符串分割成字符串数组

然后用StringBuilder将其拼接起来

Java 代码

class Solution {
    public String reverseWords(String s) {
        if(s==null)
            return "";
        s = s.trim();
        if(s.length()==0)
            return s;

        String[] strs = s.split(" ");
        int len = strs.length;
        for(int i = 0; i < len/2;i++){
            String tmp = strs[i];
            strs[i] = strs[len-i-1];
            strs[len-i-1] = tmp;
        }

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < len-1; i++) {
            sb.append(strs[i]).append(" ");
        }
        sb.append(strs[len-1]);

        return sb.toString();
    }
}

算法2

1. 先反转整个字符串中的每个字符
2. 然后在将每个单词反转回来

Java 代码

class Solution {
    public String reverseWords(String s) {
        if(s==null)
            return "";
        s = s.trim();
        if(s.length()==0)
            return s;

        char[] arr = s.toCharArray();
        int len = arr.length;
        reserver(arr,0,len-1);

        int start = 0,end = 0;
        while (end!=len){
            if(arr[end]==' '){
                reserver(arr,start,end-1);
                start = end+1;
                end++;
            }else if(end==len-1){
                reserver(arr,start,end);
                break;
            }else {
                end++;
            }
        }
        return new String(arr);

    }

    private void reserver(char[] arr, int start, int end) {
        for (int i = 0; i < (end-start)/2+1; i++) {
            char tmp = arr[start+i];
            arr[start+i] = arr[end-i];
            arr[end-i] = tmp;
        }

    }
}