这里是我见过的二分查找的几种写法：

// defined l, r, m
while (l < r) {
    m = (l + r) / 2;
    if (check(m)) l = m;
    else r = m;
}
// answer is l

// defined l, r, m
while (l <= r) {
    m = (l + r) / 2;
    if (check(m)) l = m;
    else r = m - 1;
}
if (check(l))  // answer is l
else // answer is r

// defined l, r, m, a
while (l <= r) {
    m = (l + r) / 2;
    if (check(m)) {
        a = m;
        l = m + 1;
    } else {
        r = m - 1;
    }
}
// answer is a

请问：这三种写法有何异同？一般推荐使用哪种写法？