Leetcode 443. String Compression

Given an array of characters, compress it in-place.

The length after compression must always be smaller than or equal to the original array.

Every element of the array should be a character (not int) of length 1.

After you are done modifying the input array in-place, return the new length of the array.

Follow up:
Could you solve it using only O(1) extra space?

Example 1:

Input:
["a","a","b","b","c","c","c"]

Output:
Return 6, and the first 6 characters of the input array should be: ["a","2","b","2","c","3"]

Explanation:
"aa" is replaced by "a2". "bb" is replaced by "b2". "ccc" is replaced by "c3".

Example 2:

Input:
["a"]

Output:
Return 1, and the first 1 characters of the input array should be: ["a"]

Explanation:
Nothing is replaced.

Example 3:

Input:
["a","b","b","b","b","b","b","b","b","b","b","b","b"]

Output:
Return 4, and the first 4 characters of the input array should be: ["a","b","1","2"].

Explanation:
Since the character "a" does not repeat, it is not compressed. "bbbbbbbbbbbb" is replaced by "b12".
Notice each digit has it's own entry in the array.

Note:

1. All characters have an ASCII value in [35, 126].
2. 1 <= len(chars) <= 1000.

Code (Java):

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class Solution {     public int compress(char[] chars) {         if (chars == null || chars.length == 0) {             return 0;         }                   int newPos = 0;         int count = 1;         for (int i = 0; i < chars.length; i++) {             count = 1;             while (i < chars.length && i != chars.length - 1 && (chars[i] == chars[i + 1])) {                 count++;                 i++;             }               int numChars = getNumberofChars(count);             newPos = copyArray(chars, newPos, chars[i], count, numChars);         }                   return newPos;     }       private int copyArray(char[] chars, int pos, char c, int count, int numChars) {         chars[pos] = c;         pos++;           if (count > 1) {             int end = pos + numChars - 1;             while (count > 0) {                 int digit = count % 10;                 chars[end] = (char)(digit + '0');                 end--;                   count /= 10;             }               pos += numChars;         }           return pos;     }       private int getNumberofChars(int num) {         int ans = 0;         if (num < 10) {             ans = 1;         } else if (num < 100) {             ans = 2;         } else if (num < 1000) {             ans = 3;         } else {             ans = 4;         }           return ans;     } }