55. Length of Last Word

Walk backward from the end of the string. Skip any trailing spaces, then count characters until you hit the next space or the start — that count is the last word's length.

Idea

The last word is at the tail of the string, possibly with trailing spaces after it. Scanning from the right finds it in one pass without splitting the whole string: first skip trailing spaces, then count the run of non-space characters.

Algorithm

1. Start i at the last index. While s[i] is a space, decrement i (skip trailing spaces).
2. Then, while i >= 0 and s[i] is not a space, increment a length counter and decrement i.
3. Return length.

Worked example: " fly me to the moon "

• Skip the two trailing spaces → land on n.
• Count n, o, o, m → length 4; the next character is a space, stop.

Result: 4 (the word moon).

Complexity

• Time: O(n) worst case, but only touches the trailing spaces and last word.
• Space: O(1).

class Solution {
    public int lengthOfLastWord(String s) {
        int i = s.length() - 1;
        while (i >= 0 && s.charAt(i) == ' ') {
            i--;
        }
        int length = 0;
        while (i >= 0 && s.charAt(i) != ' ') {
            length++;
            i--;
        }
        return length;
    }
}