Remove leading/trailing/ and duplicate space with one remaining

Problem:

Remove leading/trailing/ and duplicate space with one remaining

//input "_ _ _ you_ _ _get_ _ _ offer_ _ _" , output "you_get_offer"

Solution:

s1: keep the space after the word;

public String removeSpace(String str){
    if(str == null || str.length == 0) return null;
    char[] array = str.toCharArray();
    int slow = 0;
    for(int fast = 0; fast < array.length; fast++){
        if(array[fast] != ' ' || fast != 0 && array[fast - 1] != ' '){
            array[slow++] = array[fast];
        } 
    }
    if(slow == 0) return '';
    return array[slow-1] ==' ' ? new String(array, 0, slow-1) : new String(array, 0, slow);
}