Sort Transformed Array -- LeetCode

Given a sorted array of integers nums and integer values a, b and c. Apply a 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 = [-, -, , ], a = , b = , c = ,

Result: [, , , ]

nums = [-, -, , ], a = -, b = , c = 

Result: [-, -, , ]

思路：因为是排好序的数组，我们用两个指针从数组两边向中间计算答案。根据每次计算的函数值的大小决定移动哪个指针。复杂度O(N)。

 class Solution {
 public:
     int f(int x, int a, int b, int c) {
         return a * x * x + b * x + c;
     }
     vector<int> sortTransformedArray(vector<int>& nums, int a, int b, int c) {
         int len = nums.size();
         vector<int> res(len);
         int left = , right = nums.size() - , count = ;
         while (left <= right) {
             int leftRes = f(nums[left], a, b, c);
             int rightRes = f(nums[right], a, b, c);
             bool goLeft = (a >=  && leftRes >= rightRes) || (a <  && leftRes <= rightRes);
             int curPos = (a >=  ? len -  - count : count);
             if (goLeft) {
                 res[curPos] = leftRes;
                 left++;
             } else {
                 res[curPos] = rightRes;
                 right--;
             }
             count++;
         }
         return res;
     }
 };