翻转单词顺序列

例如，"student. a am I"，正确的句子应该是 "I am a student."

解题思路

先使用空格将字符串切割为字符串数组，再重新拼接新的字符串

public class Solution {
    public String ReverseSentence(String str) {
        if(str == null || str.trim().length() == 0) {
            return str;
        }
        String[] strs = str.split(" ");
        StringBuilder sb = new StringBuilder();
        for(int i = strs.length - 1; i >= 0; i--) {
            sb.append(strs[i]);
            if(i != 0) {
                sb.append(" ");
            }
        }
        return sb.toString();
    }
}

第二种思路，先翻转整个句子，然后，依次翻转每个单词，依据空格来确定单词的起始和终止位置

public class Solution {
    
    public String ReverseSentence(String str) {
        if(str == null || str.trim().length() == 0) {
            return str;
        }
        char[] chars = str.toCharArray();
        reverse(chars, 0, chars.length - 1);
        int blank = -1;
        for(int i = 0; i < chars.length; i++) {
            if(chars[i] == ' ') {
                int nextBlank = i;
                reverse(chars, blank + 1, nextBlank - 1);
                blank = nextBlank;
            }
        }
        // 对最后一个单词单独进行反转
        reverse(chars, blank + 1, chars.length - 1);
        return new String(chars);
    }

    public void reverse(char[] chars, int low, int high){
        while(low < high){
            char temp = chars[low];
            chars[low] = chars[high];
            chars[high] = temp;
            low++;
            high--;
        }
    }
}