Reverse words in a string

Split on whitespace to get the words, reverse their order with two pointers, then rejoin with single spaces.

The idea

Splitting with RemoveEmptyEntries throws away the extra spaces in one step, so leading, trailing and doubled spaces never need special-case code. What is left is an ordinary array reversal with two pointers.

Code

public static string ReverseWords(string s) {
    var words = s.Split(' ', StringSplitOptions.RemoveEmptyEntries);
    int l = 0, r = words.Length - 1;
    while (l < r) { (words[l], words[r]) = (words[r], words[l]); l++; r--; }
    return string.Join(' ', words);
}

Tips

• string.Join(' ', words) puts exactly one space between words, cleaning up the original spacing.
• The split and the reversal are two separate, simple passes: do not try to do both at once.

In the .NET library

Array.Reverse(words) is the built-in for the middle step; writing it out with l and r makes the pointer movement visible.