#165比较版本号

final class Solution {
    public int compareVersion(String version1, String version2) {
        int first = 0;
        int second = 0;
        // 每轮同步比较一个 revision，缺失的尾段按空的零段处理。
        while (first < version1.length() || second < version2.length()) {
            int firstEnd = nextDot(version1, first);
            int secondEnd = nextDot(version2, second);
            // 跳过前导零后，剩余长度就是该段数值的有效位数。
            while (first < firstEnd && version1.charAt(first) == '0') first++;
            while (second < secondEnd && version2.charAt(second) == '0') second++;

            int firstLength = firstEnd - first;
            int secondLength = secondEnd - second;
            // 有效位数不同可直接判断大小，无需解析为可能溢出的整数。
            if (firstLength != secondLength) return firstLength > secondLength ? 1 : -1;
            // 位数相同时按高位到低位比较，等价于比较十进制数值。
            for (int offset = 0; offset < firstLength; offset++) {
                char left = version1.charAt(first + offset);
                char right = version2.charAt(second + offset);
                if (left != right) return left > right ? 1 : -1;
            }
            first = firstEnd + 1;
            second = secondEnd + 1;
        }
        return 0;
    }

    private int nextDot(String version, int start) {
        int index = start;
        while (index < version.length() && version.charAt(index) != '.') index++;
        return index;
    }
}