360. Sort Transformed Array

If a >=0, the minimum value is at the array's vertex. So we need to move the two end pointers toward the vertex and output from right to left.
If a <0, the maximum value is at the array's vertex. So we need to move the two end pointers toward the vertex but output from left to right.

1:  class Solution {  
2:  public:  
3:    vector<int> sortTransformedArray(vector<int>& nums, int a, int b, int c) {  
4:      int start = 0, end = nums.size()-1;  
5:      int i = a >= 0 ? nums.size()-1 : 0;  
6:      vector<int> res(nums.size(), 0);  
7:      while (start <= end) {  
8:        int startNum = computeNumber(nums[start], a, b, c);  
9:        int endNum = computeNumber(nums[end], a, b, c);  
10:        if (a >= 0) {  
11:          if (startNum >= endNum) { res[i--] = startNum; start++; }  
12:          else { res[i--] = endNum; end--; }  
13:        } else {  
14:          if (startNum <= endNum) { res[i++] = startNum; start++; }  
15:          else { res[i++] = endNum; end--; }  
16:        }  
17:      }  
18:      return res;  
19:    }  
20:    int computeNumber(int n, int a, int b, int c) {  
21:      return a*n*n+b*n+c;  
22:    }  
23:  };