LeetCode 151 - Reverse Words in a String

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

For example,
Given s = "the sky is blue",
return "blue is sky the".

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

Clarification:

• What constitutes a word?
  A sequence of non-space characters constitutes a word.
• Could the input string contain leading or trailing spaces?
  Yes. However, your reversed string should not contain leading or trailing spaces.
• How about multiple spaces between two words?
  Reduce them to a single space in the reversed string.

Solution 1: two-pass.

public String reverseWords(String s) {
    String[] words = s.trim().split("\\s+");
	if(words.length == 0) {
        return "";
    }
	StringBuilder sb = new StringBuilder(words[words.length-1]);
	for(int i=words.length-2; i >=0; i--) {
		sb.append(" "+words[i]);
	}
	return sb.toString();
}

 

Solution 2: one-pass.

We can do better in one-pass. While iterating the string in reverse order, we keep track of a word’s begin and end position. When we are at the beginning of a word, we append it.

public String reverseWords(String s) {
    StringBuilder sb = new StringBuilder();
    int end = s.length();
    int i = end-1;
    while(i>=0) {
        if(s.charAt(i) == ' ') {
            if(i < end-1) {
                sb.append(s.substring(i+1, end)).append(" ");
            }
            end = i;
        }
        i--;
    }
    sb.append(s.substring(i+1, end));
    return sb.toString().trim();
}