158. Read N Characters Given Read4 II - Call multiple times

跟157. Read N Characters Given Read4的区别是前一次read过的剩余字符下次read的时候直接写进buf

/**
 * The read4 API is defined in the parent class Reader4.
 *     int read4(char *buf4);
 */

class Solution {
public:
    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */
    int read(char *buf, int n) {
        int cnt = 0;
        while (b != e && cnt < n) {
            buf[cnt++] = buf4[b++];
        }
        if (cnt == n) return cnt;
        b = 0; // cnt不等于n，说明b == e，重置b，更新e读新字符
        e = read4(buf4);
        if (e == 0) return cnt;
        return cnt + read(buf + cnt, n - cnt);
    }

    char buf4[4];
    int b = 0, e = 0;
};

// Forward declaration of the read4 API.
int read4(char *buf);

class Solution {
public:
    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */
    int read(char *buf, int n) {
        int cnt = 0;
        while (i < e && cnt < n) { // 在全局buffer的边界之内尽可能多的读字符
            buf[cnt++] = t[i++];
        }
        if (i == e) i = e = 0; // 如果全局buffer里到边界的所有字符都被读走，则重置下标和边界
        if (cnt == n) return cnt; // 如果已经读够了需要的字符，则返回
        e = read4(t);
        if (e == 0) return cnt; // 如果还需要继续读字符但是读出的字符数为0
        return cnt + read(buf + cnt, n - cnt); // 如果还需要继续读字符，递归
    }

    int i = 0, e = 0; // 维护全局buffer的下标和右边界（不一定为4）
    char t[4];
};