字符串原地修改双指针经典实现

字符串原地修改经常遇到的一类题，双指针一个用于写入，一个用于扫描，互不干扰，各司其职。
这类双指针可以模拟栈，很多情况下比真正的栈要更灵活好用。

题目：https://leetcode.cn/problems/reverse-words-in-a-string/

    string reverseWords(string s) {
        reverse(s.begin(), s.end());
        int write = 0, scan = 0;
        while (scan <= s.size()) {
            if (scan == s.size() || s[scan] == ' ') {
                // 屏蔽开头是空格情况
                if (write == 0) {
                    scan++;
                    continue;
                }
                // 反转单词
                int w = write - 1;
                while (w >= 0 && s[w] != ' ') w--;
                reverse(s.begin() + w + 1, s.begin() + write);
                // scan后面没有有效单词退出
                while (scan < s.size() && s[scan] == ' ') scan++;
                if (scan == s.size()) break;
                // 单词之间补充空格
                s[write++] = ' ';
            } else {
                s[write++] = s[scan++];
            }
        }
        return s.substr(0, write);
    }

题目：https://leetcode.cn/problems/asteroid-collision/
栈模拟

    vector<int> asteroidCollision(vector<int>& asteroids) {
        int write = 0, scan = 0;
        while (scan < asteroids.size()) {
            if (write == 0 || !(asteroids[scan] < 0 && asteroids[write - 1] > 0)) {
                asteroids[write++] = asteroids[scan++];
            } else if (abs(asteroids[scan]) > abs(asteroids[write - 1])) {
                write--;
            } else if (abs(asteroids[scan]) == abs(asteroids[write - 1])) {
                write--;
                scan++;
            } else {
                scan++;
            }
        }
        while (asteroids.size() > write) {
            asteroids.pop_back();
        }
        return asteroids;
    }

题目：https://leetcode.cn/problems/string-compression/

    int compress(vector<char>& chars) {
        int write = 0, i = 0;
        while (i < chars.size()) {
            if (i + 1 < chars.size() && chars[i] == chars[i + 1]) {
                chars[write++] = chars[i++];
                int count = 2;
                while (i + 1 < chars.size() && chars[i] == chars[i + 1]) {
                    i++;
                    count++;
                }
                i++;
                stack<char> s;
                while (count) {
                    s.push('0' + count % 10);
                    count /= 10;    
                }
                while (!s.empty()) {
                    chars[write++] = s.top();
                    s.pop();
                }
            } else {
                chars[write++] = chars[i++];
            }
        }
        return write;
    }

题目：https://leetcode.cn/problems/removing-stars-from-a-string/

    string removeStars(string s) {
        int write = 0, scan = 0;
        while (scan < s.size()) {
            if (s[scan] == '*') {
                write--;
                scan++;
                continue;
            }
            s[write++] = s[scan++];
        }
        return s.substr(0, write);
    }