LeetCode in Net

151. Reverse Words in a String

Medium

Given an input string s, reverse the order of the words.

A word is defined as a sequence of non-space characters. The words in s will be separated by at least one space.

Return a string of the words in reverse order concatenated by a single space.

Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.

Example 1:

Input: s = “the sky is blue”

Output: “blue is sky the”

Example 2:

Input: s = “ hello world “

Output: “world hello”

Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3:

Input: s = “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.

Example 4:

Input: s = “ Bob Loves Alice “

Output: “Alice Loves Bob”

Example 5:

Input: s = “Alice does not even like bob”

Output: “bob like even not does Alice”

Constraints:

• 1 <= s.length <= 104
• s contains English letters (upper-case and lower-case), digits, and spaces ' '.
• There is at least one word in s.

Follow-up: If the string data type is mutable in your language, can you solve it in-place with O(1) extra space?

Solution

using System.Text;

public class Solution {
    public string ReverseWords(string s) {
        var sb = new StringBuilder();
        int i = s.Length - 1;
        while (i >= 0) {
            if (s[i] == ' ') {
                i--;
                continue;
            }
            int start = s.LastIndexOf(' ', i);
            sb.Append(' ');
            sb.Append(s, start + 1, i - start);
            i = start - 1;
        }
        if (sb.Length > 0) {
            sb.Remove(0, 1);
        }
        return sb.ToString();
    }
}

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