Given a sorted array of integers nums and integer values a, b and c. Apply a quadratic function of the form f(x) = ax2 + bx + c to each element x in the array.

The returned array must be in sorted order.

Expected time complexity: O(n)

Example:

nums = [-4, -2, 2, 4], a = 1, b = 3, c = 5,

Result: [3, 9, 15, 33]

nums = [-4, -2, 2, 4], a = -1, b = 3, c = 5

Result: [-23, -5, 1, 7]

 1 public class Solution {
 2     public int[] SortTransformedArray(int[] nums, int a, int b, int c) {
 3         if (nums == null || nums.Length < 2) return nums;
 4         
 5         var result = new int[nums.Length];
 6         int i = 0, j = nums.Length - 1;
 7         int index = a >= 0
 
 ? j : i;
 8         
 9         while (i <= j)
10         {
11             var q1 = Quad(nums[i], a, b, c);
12             var q2 = Quad(nums[j], a, b, c);
13             
14             if (a >= 0)
15             {
16                 if (q1 >= q2)
17                 {
18                     i++;
 
19                     result[index--] = q1;
20                 }
21                 else
22                 {
23                     j--;
24                     result[index--] = q2;
25                 }
26             }
27             else
28             {
29                 if (q1 <= q2)
30                 {
31                     i++;
32                     result[index++] = q1;
33                 }
34                 else
35                 {
36                     j--;
37                     result[index++] = q2;
38                 }
39             }
40         }
41         
42         return result;
43     }
44     
45     private int Quad(int x, int a, int b, int c) {
46         return a * x * x + b * x + c;
47     }
48 }

Leetcode 360: Sort Transformed Array