对于大于等于 K 的值，给定二次方程的最小根

给定二次方程F(x) = Ax ² + Bx + C的常数为A、B 和 C以及整数K ，任务是找到根x的最小值，使得F(x) ≥ K 。如果不存在这样的值，则打印“-1” 。假设F(x)是一个单调递增的函数。

例子：

Input: A = 3, B = 4, C = 5, K = 6
Output: 1
Explanation:
For the given values F(x) = 3x² + 4x + 5 the minimum value of x is 1, F(x) = 12, which is greater than the given value of K.

Input: A = 3, B = 4, C = 5, K = 150
Output: 7
Explanation: 
For the given values F(x) = 3x² + 4x + 5 the minimum value of x is 7, F(x) = 180, which is greater than the given value of K.

方法：想法是使用二分搜索来找到x的最小值。以下是步骤：

• 要获得等于或大于K的值， x的值必须在[1, sqrt(K)]范围内，因为这是一个二次方程。
• 现在，基本上需要在范围内搜索适当的元素，因此实现了这种二分搜索。
• 计算F(mid) ，其中mid是范围[1, sqrt(K)]的中间值。现在可能出现以下三种情况：
  • 如果F(mid) ≥ K && F(mid) < K：这意味着当前的 mid 是所需的答案。
  • 如果F(mid) < K：这意味着 mid 的当前值小于 x 的要求值。因此，向右移动，即在后半部分，因为F(x)是一个递增函数。
  • 如果F(mid) > K：这意味着 mid 的当前值大于 x 的所需值。所以，向左移动，即前半部分为F(x)是一个递增函数。

下面是上述方法的实现：

C++

// C++ program for the above approach
#include 
using namespace std;
 
// Function to calculate value of
// quadratic equation for some x
int func(int A, int B, int C, int x)
{
    return (A * x * x + B * x + C);
}
 
// Function to calculate the minimum
// value of x such that F(x) >= K using
// binary search
int findMinx(int A, int B, int C, int K)
{
 
    // Start and end value for
    // binary search
    int start = 1;
    int end = ceil(sqrt(K));
 
    // Binary Search
    while (start <= end) {
        int mid = start + (end - start) / 2;
 
        // Computing F(mid) and F(mid-1)
        int x = func(A, B, C, mid);
        int Y = func(A, B, C, mid - 1);
 
        // Checking the three cases
 
        // If F(mid) >= K  and
        // F(mid-1) < K return mid
        if (x >= K && Y < K) {
            return mid;
        }
 
        // If F(mid) < K go to mid+1 to end
        else if (x < K) {
            start = mid + 1;
        }
 
        // If F(mid) > K go to start to mid-1
        else {
            end = mid - 1;
        }
    }
 
    // If no such value exist
    return -1;
}
 
// Driver Code
int main()
{
    // Given coefficients of Equations
    int A = 3, B = 4, C = 5, K = 6;
 
    // Find minimum value of x
    cout << findMinx(A, B, C, K);
    return 0;
}

Java

// Java program for the above approach
import java.util.*;
 
class GFG{
 
// Function to calculate value of
// quadratic equation for some x
static int func(int A, int B, int C, int x)
{
    return (A * x * x + B * x + C);
}
 
// Function to calculate the minimum
// value of x such that F(x) >= K using
// binary search
static int findMinx(int A, int B, int C, int K)
{
     
    // Start and end value for
    // binary search
    int start = 1;
    int end = (int)Math.ceil(Math.sqrt(K));
 
    // Binary Search
    while (start <= end)
    {
        int mid = start + (end - start) / 2;
 
        // Computing F(mid) and F(mid-1)
        int x = func(A, B, C, mid);
        int Y = func(A, B, C, mid - 1);
 
        // Checking the three cases
        // If F(mid) >= K and
        // F(mid-1) < K return mid
        if (x >= K && Y < K)
        {
            return mid;
        }
 
        // If F(mid) < K go to mid+1 to end
        else if (x < K)
        {
            start = mid + 1;
        }
 
        // If F(mid) > K go to start to mid-1
        else
        {
            end = mid - 1;
        }
    }
     
    // If no such value exist
    return -1;
}
 
// Driver code
public static void main(String[] args)
{
     
    // Given coefficients of Equations
    int A = 3, B = 4, C = 5, K = 6;
     
    // Find minimum value of x
    System.out.println(findMinx(A, B, C, K));
}
}
 
// This code is contributed by offbeat

Python3

# Python3 program for the above approach
import math
 
# Function to calculate value of
# quadratic equation for some x
def func(A, B, C, x):
     
    return (A * x * x + B * x + C)
 
# Function to calculate the minimum
# value of x such that F(x) >= K using
# binary search
def findMinx(A, B, C, K):
 
    # Start and end value for
    # binary search
    start = 1
    end = math.ceil(math.sqrt(K))
 
    # Binary Search
    while (start <= end):
        mid = start + (end - start) // 2
 
        # Computing F(mid) and F(mid-1)
        x = func(A, B, C, mid)
        Y = func(A, B, C, mid - 1)
 
        # Checking the three cases
 
        # If F(mid) >= K and
        # F(mid-1) < K return mid
        if (x >= K and Y < K):
            return mid
         
        # If F(mid) < K go to mid+1 to end
        elif (x < K):
            start = mid + 1
         
        # If F(mid) > K go to start to mid-1
        else:
            end = mid - 1
     
    # If no such value exist
    return -1
 
# Driver Code
 
# Given coefficients of Equations
A = 3
B = 4
C = 5
K = 6
 
# Find minimum value of x
print(findMinx(A, B, C, K))
 
# This code is contributed by code_hunt

C#

// C# program for the above approach
using System;
class GFG{
 
// Function to calculate value of
// quadratic equation for some x
static int func(int A, int B, int C, int x)
{
    return (A * x * x + B * x + C);
}
 
// Function to calculate the minimum
// value of x such that F(x) >= K using
// binary search
static int findMinx(int A, int B, int C, int K)
{
     
    // Start and end value for
    // binary search
    int start = 1;
    int end = (int)Math.Ceiling(Math.Sqrt(K));
 
    // Binary Search
    while (start <= end)
    {
        int mid = start + (end - start) / 2;
 
        // Computing F(mid) and F(mid-1)
        int x = func(A, B, C, mid);
        int Y = func(A, B, C, mid - 1);
 
        // Checking the three cases
        // If F(mid) >= K and
        // F(mid-1) < K return mid
        if (x >= K && Y < K)
        {
            return mid;
        }
 
        // If F(mid) < K go to mid+1 to end
        else if (x < K)
        {
            start = mid + 1;
        }
 
        // If F(mid) > K go to start to mid-1
        else
        {
            end = mid - 1;
        }
    }
     
    // If no such value exist
    return -1;
}
 
// Driver code
public static void Main(String[] args)
{
     
    // Given coefficients of Equations
    int A = 3, B = 4, C = 5, K = 6;
     
    // Find minimum value of x
    Console.WriteLine(findMinx(A, B, C, K));
}
}
 
// This code is contributed by sapnasingh4991

Javascript

 
// javascript program for the above approach
 
// Function to calculate value of
// quadratic equation for some x
function func(A , B , C , x)
{
    return (A * x * x + B * x + C);
}
 
// Function to calculate the minimum
// value of x such that F(x) >= K using
// binary search
function findMinx(A , B , C , K)
{
     
    // Start and end value for
    // binary search
    var start = 1;
    var end = parseInt(Math.ceil(Math.sqrt(K)));
 
    // Binary Search
    while (start <= end)
    {
        var mid = start + parseInt((end - start) / 2);
 
        // Computing F(mid) and F(mid-1)
        var x = func(A, B, C, mid);
        var Y = func(A, B, C, mid - 1);
 
        // Checking the three cases
        // If F(mid) >= K and
        // F(mid-1) < K return mid
        if (x >= K && Y < K)
        {
            return mid;
        }
 
        // If F(mid) < K go to mid+1 to end
        else if (x < K)
        {
            start = mid + 1;
        }
 
        // If F(mid) > K go to start to mid-1
        else
        {
            end = mid - 1;
        }
    }
     
    // If no such value exist
    return -1;
}
 
// Driver code
 
// Given coefficients of Equations
var A = 3, B = 4, C = 5, K = 6;
 
// Find minimum value of x
document.write(findMinx(A, B, C, K));
 
// This code is contributed by shikhasingrajput

输出：

1

时间复杂度： O(log(sqrt(K))
辅助空间： O(1)