面试题 01.06. 字符串压缩 

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class Solution {
    public String compressString(String S) {
        StringBuilder sb = new StringBuilder();

        int i = 0, j = 0;
        while (j < S.length()) {
            while (j < S.length() && S.charAt(j) == S.charAt(i)) {
                j++;
            }

            // now: j == S.length() or S[i] != S[j]
            sb.append(S.charAt(i)).append(j - i);
            i = j;
        }

        return sb.length() < S.length() ? sb.toString() : S;
    }
}

References

面试题 01.06. 字符串压缩