Description

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.

Solutions

1. 不优雅的拼凑

  自己实现的版本真的是一堆 if-else,说明没有对规律了解清楚:

# Time Complexity: O(n)
# Space Complexity: O(1)
class Solution:
    def compress(self, chars: List[str]) -> int:
        if not chars:
            return 0
        if len(chars) == 1:
            return 1
        n = len(chars)
        pre = chars[0]
        cnt = 1
        idx = 0
        res = ''
        for i in range(1, n):
            if chars[i] == pre:
                cnt += 1
            else:
                chars[idx] = pre
                pre = chars[i]
                if cnt == 1:
                    idx += 1
                elif cnt >= 10:
                    for c in str(cnt):
                        idx += 1
                        chars[idx] = c
                    idx += 1
                else:
                    chars[idx+1] = str(cnt)
                    idx += 2
                cnt = 1
            if i == n-1:
                chars[idx] = pre
                if cnt == 1:
                    idx += 1
                elif cnt >= 10:
                    for c in str(cnt):
                        idx += 1
                        chars[idx] = c
                    idx += 1
                else:
                    chars[idx+1] = str(cnt)
                    idx += 2
        return idx
# Runtime: 56 ms, faster than 98.75% of Python3 online submissions for String Compression.
# Memory Usage: 12.9 MB, less than 100.00% of Python3 online submissions for String Compression.

2. 优雅的版本

# Time Complexity: O(n)
# Space Complexity: O(1)
class Solution:
    def compress(self, chars: List[str]) -> int:
        n = len(chars)
        i, count = 0, 1
        for j in range(1,n+1):
            if j < n and chars[j] == chars[j-1]:
                count += 1
            else:
                chars[i] = chars[j-1]
                i += 1
                if count > 1:
                    for m in str(count):
                        chars[i] = m
                        i += 1
                count = 1
        return i
# Runtime: 60 ms, faster than 95.41% of Python3 online submissions for String Compression.
# Memory Usage: 12.8 MB, less than 100.00% of Python3 online submissions for String Compression.

References

  1. 443. String Compression