String Compression (Run-Length)

Run-length-encode a character array in place: `[a,a,b,b,c,c,c]` becomes `[a,2,b,2,c,3]`. Goldman's interviewers consistently expect the in-place version, not a fresh-allocation string output. Requirements Modify a character array in place so that consecutive runs are replaced by <char><count>. A run of length 1 is written as just the character, with no 1 suffix. A run of length 10 or longer is written as the character followed by each digit of the count as a separate cell ([b,1,2] for 12 bs). Return the new length of the array; trailing cells may contain arbitrary content. public int compress(char[] chars) Examples Input: ['a','a','b','b','c','c','c'] Output: 6 — array prefix becomes ['a','2','b','2','c','3', ...] Input: ['a'] Output: 1 — array prefix is ['a'] Input: ['a','b','b','b','b','b','b','b','b','b','b','b','b'] Output: 4 — array prefix becomes ['a','b','1','2'] Notes Two-pointer (read, write) is the canonical solution. read scans the array; for each run, write emits the character and then the count digits. O(n) time, O(1) extra space. The hidden trap is the run-length-≥10 case: writing each digit individually requires either building a temporary buffer or writing in reverse and then reversing in place. Goldman's hidden tests do include long runs (≥10); a solution that hardcodes single-digit counts fails silently. Watch the trailing pointer — return write, not chars.length. Preparation Implement the in-place two-pointer form and verify the long-run case by hand. LC 443 "String Compression" is the canonical equivalent. The Goldman-tagged set on LeetCode features this problem prominently.