数据结构——二分搜索

递归实现

/**
 * 二分搜索（递归实现）
 * @param A     待搜索数组
 * @param num   搜索值
 * @param left  区间左指针
 * @param right 区间右指针
 * @param mid   基准
 */
private static void search(int[] A, int num, int left, int right, int mid) {
    if (A[mid] == num) { // 如果找到要找的数，记录其下标
        res = mid;
    }
    if (left >= right)
        return ;
    if (A[mid] >= num) { // 出现在左边
        right = mid - 1;
        mid = left + (right-left)/2;
        search(A, num, left, right, mid);
    }
    if (A[mid] < num) { // 出现在右边
        left = mid + 1;
        mid = left + (right-left)/2;
        search(A, num, left, right, mid);
    }
}

非递归实现

/**
 * 二分搜索（非递归实现）
 * @param A     带搜索数组
 * @param n     数组长度
 * @param num   搜索值
 * @return
 */
private static int search2(int[] A, int n, int num) {
    int left = 0;
    int right = n - 1;
    int res = -1;
    while (left <= right) {
        int mid = left + (right - left)/2;
        if (A[mid] == num) {
            res = mid;
            right = mid - 1;
        }
        if (A[mid] >= num) { // 左区间
            right = mid - 1;
        } else {
            left = mid + 1;
        }
    }
    return res;
}

private static int search3(int[] arr, int n, int num) {
    int left = 0;
    int right = n-1;
    while (left <= right) {
        int mid = left + ((right-left)>>1);
        if (arr[mid] > num) {
            right = mid-1;
        }else if (arr[mid] < num) {
            left = mid+1;
        }else {
            return mid;
        }
    }
    return -1;
}