二分法变体

二分法真的不那么简单，主要利用边界变换，实现了二分法的各个变种。

这里假定查找区按升序排列（arr数组成员递增）
1.找出第一个与key相等的元素

int *binary_search(int *arr, int key,int n)
{
    int left = 0, right = n-1;
    while(left<=right) {
        int mid = (left + right)/2;
        if(arr[mid] >= key) right = mid - 1;
        else left = mid + 1;
    }
    if(left < n && arr[left] == key)
        return &arr[left];
    return NULL;
}

2.找出最后一个与key相等的元素

int *binary_search(int *arr, int key,int n)
{
    int left = 0, right = n-1;
    while(left<=right) {
        int mid = (left + right)/2;
        if(arr[mid] > key) right = mid - 1;
        else left = mid + 1;
    }
    if(right >= 0 && arr[right] == key)
        return &arr[right];
    return NULL;
}

1. 查找第一个等于或者大于Key的元素

int *binary_search(int *arr, int key,int n)
{
    int left = 0, right = n-1;
    while(left<=right) {
        int mid = (left + right)/2;
        if(arr[mid] >= key) right = mid - 1;
        else left = mid + 1;
    }
    if(left > n)
        return NULL;
    return &arr[left];
}

4.查找第一个大于key的元素

int *binary_search(int *arr, int key,int n)
{
    int left = 0, right = n-1;
    while(left<=right) {
        int mid = (left + right)/2;
        if(arr[mid] > key) right = mid - 1;
        else left = mid + 1;
    }
    if(left > n)
        return NULL;
    return &arr[left];
}

5.查找最后一个等于或者小于key的元素

int *binary_search(int *arr, int key,int n)
{
    int left = 0, right = n-1;
    while(left<=right) {
        int mid = (left + right)/2;
        if(arr[mid] > key) right = mid - 1;
        else left = mid + 1;
    }
    if(right < 0)
        return NULL;
    return &arr[right];
}

6.查找最后一个小于key的元素

int *binary_search(int *arr, int key,int n)
{
    int left = 0, right = n-1;
    while(left<=right) {
        int mid = (left + right)/2;
        if(arr[mid] >= key) right = mid - 1;
        else left = mid + 1;
    }
    if(right < 0)
        return NULL;
    return &arr[right];
}