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.

双指针+一个常数用来计数

时间：O(N)，空间：O(1)

class Solution {
    public int compress(char[] chars) {
        int resIdx = 0, idx = 0;
        while(idx < chars.length) {
            char cur = chars[idx];
            int cnt = 0;
            while(idx < chars.length && cur == chars[idx]) {
                idx++;
                cnt++;
            }
            chars[resIdx++] = cur;
            if(cnt != 1) {
                for(char c : Integer.toString(cnt).toCharArray()) {
                    chars[resIdx++] = c;
                }
            }
        }
        return resIdx;
    }
}

二刷：

class Solution {
    public int compress(char[] chars) {
        if(chars == null || chars.length == 0) {
            return 0;
        }
        int s = 0, f = 0;
        while(f < chars.length) {
            int f_begin = f;
            while(f < chars.length && chars[f_begin] == chars[f]) {
                f++;
            }
            int cnt = f - f_begin;
            if(cnt > 1) {
                chars[s++] = chars[f_begin];
                String str = Integer.toString(cnt);
                for(int i = 0; i < str.length(); i++) {
                    chars[s++] = str.charAt(i);
                }
            } else {
                chars[s++] = chars[f_begin];
            }
        }
        return s;
    }
}