Encode String

Given a String like "ABBCCCC", encode it to A2B4C. Avoid to make the encode string larger than original string. Do it in-place! The repeat count should less than 10.

Think

• Pass the char array by reversed order so that it's easier to modify the char to count number in-place
• Index passing should from len - 2 to -1, this is tricky part to avoid the case when index is zero cannot be count
• The in-place modification index come from the len - 1, rewrite the result char by this index
• For the integer convert to character, it may need to use Character.forDigit(count, 10)
• For those rest char less than rewrite index, set them as null character "\u0000"

Solution

class Solution {
  public static void main(String[] args) {
    String str = "ABC";
    inplaceEncode(str.toCharArray());
  }

  public static void inplaceEncode(char[] chars){
    int idx = chars.length - 1;
    int count = 1;
    for(int i = chars.length - 2; i >= -1; i--) {
      if(i>=0 && chars[i] == chars[i+1])
        count++;
      else {
        char cur = chars[i+1];
        if(count > 1){
          chars[idx--] = cur;
          chars[idx--] = Character.forDigit(count, 10);
        }else
          chars[idx--] = cur;
        count = 1;
      }
    }

    // place the rest char position as null char
    while(idx>=0)
      chars[idx--] = '\u0000';
  }
}