58. Length of Last Word

Given a string s consists of upper/lower-case alphabets and empty space characters ' ', return the length of last word in the string.
If the last word does not exist, return 0.
Note: A word is defined as a character sequence consists of non-space characters only.
For example,
Given s = "Hello World",
return 5.

Solution:

after skipping inceptive spaces backwards(or trim), find first space backwards

Code1：

class Solution {
    public int lengthOfLastWord(String s) {
        if(s == null || s.length() == 0) return 0;
 
        // skip space backwards
        int back_start = s.length() - 1;
        while(back_start >= 0 && s.charAt(back_start) == ' ') back_start--;
        
        // find first first space backwards from back_start
        int space_pos = -1;
        for(int i = back_start; i >= 0; i--) {
            if(s.charAt(i) ==' ') {
                space_pos = i;
                break;
            }
        }
        return start - space_pos;
    }
}

Code2：

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

Code3：

class Solution {
    public int lengthOfLastWord(String s) {
        return s.trim().length() - s.trim().lastIndexOf(" ") - 1;
    }
}