Clarifications

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.

Thinking process I

Split the input string, skip all the substrings whose length is 0 (white spaces)

Append the substring reversely to the string builder

Mistakes

Append to the string builder, instead of insert at the front

public class Solution {
    public String reverseWords(String s) {
        String[] strs = s.split(" ");
        StringBuilder sb = new StringBuilder();

        for (int i = strs.length - 1; i >= 0; --i) {
            if (strs[i].length() == 0) continue;
            sb.append(strs[i]);
            sb.append(" ");
        }

        return sb.toString().trim();
    }
}

Thinking process II

Don't need to split the string, use two pointers to remember the start and end position of a word, and append it to string builder

public class Solution {
    public String reverseWords(String s) {
        StringBuilder result = new StringBuilder();
        int endOfWord = s.length();
        for (int i = s.length() - 1; i >= 0; --i) {
            if (s.charAt(i) == ' ') {
                endOfWord = i;
            } else if (i == 0 || s.charAt(i - 1) == ' ') {
                result.append(s.substring(i, endOfWord));
                result.append(' ');
            }
        }

        return result.toString().trim();
    }
}

or

public class Solution {
    public String reverseWords(String s) {
        StringBuilder result = new StringBuilder();
        int endOfWord = s.length();
        for (int i = s.length() - 1; i >= 0; --i) {
            if (s.charAt(i) == ' ') {
                endOfWord = i;
            } else if (i == 0 || s.charAt(i - 1) == ' ') {
                if (result.length() != 0) {
                    result.append(' ');
                }
                result.append(s.substring(i, endOfWord));
            }
        }

        return result.toString();
    }
}

Follow ups - LC 186 Reverse Words in a String II

public class Solution {
    public void reverseWords(char[] s) {
        reverse(s, 0, s.length - 1);
        int startOfWord = 0;
        for (int i = 0; i <= s.length; i++) {
            if (i == s.length || s[i] == ' ') {
                reverse(s, startOfWord, i - 1);
                startOfWord = i + 1;
            }
        }
    }

    private void reverse(char[] s, int start, int end) {
        while (start < end) {
            char tmp = s[start];
            s[start] = s[end];
            s[end] = tmp;

            start++;
            end--;
        }
    }
}