查找序列中第一个满足check()条件的值

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int check(int x) {
    return x >= 5; // 举例,寻找第一个大于等于5的数
}
int bsearch(int *arr, int n) {
    int l = 0, r = n - 1;
    while (l <= r) {
        //int mid = l + ((r-l)>>1); // 避免溢出
        int mid = l + r >> 1;
        if (check(arr[mid]))
            r = mid - 1;
        else
            l = mid + 1;
    }
    return l;
}
int bsearch2(int *arr, int n) {
    int l = 0, r = n - 1;
    while (l < r) {
        int mid = l + r >> 1;
        if (check(arr[mid]))
            r = mid;
        else
            l = mid + 1;
    }
    return l;
}

注意:这样的代码是错误的(?):

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int bsearch3(int *arr, int n) {
    int l = 0, r = n - 1;
    while (l < r) {
        //int mid = l + ((r-l+1)>>1); // 避免溢出
        int mid = l + r + 1 >> 1;
        if (check(arr[mid]))
            r = mid - 1; // mid被排除在区间外!
        else
            l = mid;
    }
    return l;
}

尽管从边界处理上没有问题,但是r=mid-1导致mid被排除在区间外,并且无法再次被两个指针的任何一个指向.

查找某个具体的值是否存在

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int bsearch(int *arr, int n, int key) {
    int l = 0, r = n - 1;
    while (l <= r) {
        //int mid = l + ((r-l+1)>>1); // 避免溢出
        int mid = l + r >> 1;
        if (arr[mid] == key)
            return mid;
        else if (arr[mid] > key)
            r = mid - 1;
        else
            l = mid + 1;
    }
    return -1;
}
int bsearch2(int *arr, int n, int key) {
    int l = 0, r = n - 1;
    while (l < r) {
        //int mid = l + ((r-l+1)>>1); // 避免溢出
        int mid = l + r + 1 >> 1;
        if (arr[mid] == key)
            return mid;
        else if (arr[mid] > key)
            r = mid - 1;
        else
            l = mid;
    }
    // 判断是否相等
    if (arr[l] == key)
        return l;
    else
        return -1;
}
int bsearch3(int *arr, int n, int key) {
    int l = 0, r = n - 1;
    while (l < r) {
        //int mid = l + ((r-l+1)>>1); // 避免溢出
        int mid = l + r >> 1;
        if (arr[mid] >= key)
            r = mid;
        else
            l = mid + 1;
    }
    // 判断是否相等
    if (arr[l] == key)
        return l;
    else
        return -1;
}