public class StringReplace {

    public static void replaceSpace(String[] str, int length) {

        if(str == null || length <= 0) {

            return;

        }

        int originalLength = 0;

        int numberOfBlank = 0;

        int i = 0;

        while(str[i] != null) {

            originalLength ++;

            if(str[i] == " ") {

                numberOfBlank ++;

            }

            i++;

        }

        //新的字符串长度

        int newLength = originalLength + 2 * numberOfBlank;

        if(newLength > length) {//不能超出字符数组的长度

            return;

        }

        int originalIndex = originalLength-1;

        int newIndex = newLength-1;

        while(originalIndex >=0 && originalIndex < newIndex) {

            if(str[originalIndex] == " ") {

                str[newIndex--] = "0";

                str[newIndex--] = "2";

                str[newIndex--] = "%";

                originalIndex --;

            }else {

                str[newIndex--] = str[originalIndex--];

            }

        }

    }

    public static void main(String[] args) {

        String[] temp = {"we", " ","are", " ", "happy",null,null,null,null,null,null,null,null };

        replaceSpace(temp,13);

        for(int i=0; i< temp.length; i ++) {

            System.out.print(temp[i]);

        }

    }

}