157. Read N Characters Given Read4

iterative

// 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) {
        char t[4];
        int i = 0, cnt = 0;
        do {
            cnt = read4(t);
            int j = 0;
            while (i < n && j < cnt) {
                buf[i++] = t[j++];
            }
        } while (i < n && cnt == 4); // 注意如果read4读出来的字符少于4个就终止
        return i;
    }
};

recursive

// 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) {
        char t[4];
        int cnt = read4(t), i = 0, j = 0;
        if (cnt == 0) return 0;
        while (i < n && j < cnt) {
            buf[i++] = t[j++];
        }
        if (i == n) return n;
        return cnt + read(buf + cnt, n - cnt);
    }
};