Write a method to replace all spaces in a string with '%20'. You may assume that the string has sufficient space at the end to hold the additional characters,and that you are given the "true" length of the string. (Note: If implementing in Java,please use a character array so that you can perform this operation in place.)

Example 1:

Input: "Mr John Smith ", 13

Output: "Mr%20John%20Smith"

Example 2:

Input: "               ", 5

Output: "%20%20%20%20%20"

Note:

1. 0 <= S.length <= 500000