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.

Follow up:

For C programmers, try to solve it in-place in O(1) extra space.

two pointers

time: O(n), space: O(1)

class Solution {
    public String reverseWords(String s) {
        if(s == null || s.length() == 0) {
            return "";
        }
        char[] str = reverse(s.toCharArray(), 0, s.length() - 1);
        int start = 0, end = 0;
        for(int i = 0; i < str.length; i++) {
            if(str[i] != ' ') {
                str[end++] = str[i];
            } else if(i > 0 && str[i] == ' ' && str[i - 1] != ' ') {
                reverse(str, start, end - 1);
                str[end++] = ' ';
                start = end;
            }
        }
        reverse(str, start, end - 1);   // last word
        
        return end > 0 && str[end - 1] == ' ' ? new String(str, 0, end - 1) : new String(str, 0, end);
    }
    
    private char[] reverse(char[] c, int i, int j) {
        while(i < j) {
            char tmp = c[i];
            c[i] = c[j];
            c[j] = tmp;
            i++;
            j--;
        }
        return c;
    }
}