【刷题-LeetCode】151 Reverse Words in a String

151. Reverse Words in a String

Given an input string, reverse the string word by word.

Example 1:

Input: "the sky is blue"
Output: "blue is sky the"

Example 2:

Input: "  hello world!  "
Output: "world! hello"
Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3:

Input: "a good   example"
Output: "example good a"
Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

Note:

• A word is defined as a sequence of non-space characters.
• Input string may contain leading or trailing spaces. However, your reversed string should not contain leading or trailing spaces.
• You need to reduce multiple spaces between two words to a single space in the reversed string.

解法1 分词，然后用字符串拼接

class Solution {
public:
    string reverseWords(string s) {
        string tmp, res;
        for(int i = 0; i < s.size(); ++i){
            if(s[i] == ' '){
                if(tmp.size() > 0){
                    res = tmp + " " + res;
                    tmp = "";
                }
            }else{
                tmp += s[i];
            }
        }
        res = tmp + " " + res;
        int i = 0, j = res.size() - 1;
        while(i < s.size() && res[i] == ' ')i++;
        while(j >= 0 && res[j] == ' ')j--;
        return res.substr(i, j-i+1);
    }
};

解法2 字符串翻转。先将字符串整体翻转，然后将每个单词翻转。注意需要预处理除掉字符串首尾的空格，翻转结束后需要去除字符串中间多余的空格

class Solution {
public:
    string reverseWords(string s) {
        while(s.size() > 0 && s[0] == ' ')s.erase(0,1);
        while(s.size() > 0 && s[s.size()-1] == ' ')s.erase(s.size()-1, 1);
        reverse(s, 0, s.size()-1);
        int pre = 0, cur = 0;
        while(cur < s.size()){
            if(s[cur] == ' '){
                reverse(s, pre, cur-1);
                cur += 1;
                pre = cur;
            }else{
                cur += 1;
            }
        }
        reverse(s, pre, cur-1);
        int i = 1;
        while(i < s.size()){
            if(s[i] == ' ' && s[i-1] == ' ')s.erase(i,1);
            else i++;
        }
        return s;
    }
    void reverse(string &s, int pre, int cur){
        while(pre < cur){
            char tmp = s[pre];
            s[pre] = s[cur];
            s[cur] = tmp;
            pre++;
            cur--;
        }
    }
};