LeetCode 360. Sort Transformed Array

原题链接在这里：https://leetcode.com/problems/sort-transformed-array/description/

题目：

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]

题解：

如果a > 0, 左右两边的点就比中间的点大. two pointers夹比, 从大往小赋值.

a < 0, 左右两边的点比中间的点小. two pointers夹比, 从小往大赋值.

Time Complexity: O(n). n = nums.length.

Space: O(1). regardless res.

AC Java:

 class Solution {
     public int[] sortTransformedArray(int[] nums, int a, int b, int c) {
         if(nums == null || nums.length == 0){
             return nums;
         }

         int [] res = new int[nums.length];
         int l = 0;
         int r = nums.length-1;
         int index = a >=0 ? nums.length-1 : 0;

         while(l <= r){
             int lNum = quad(nums[l], a, b, c);
             int rNum = quad(nums[r], a, b, c);
             if(a>=0){
                 if(lNum > rNum){
                     res[index--] = lNum;
                     l++;
                 }else{
                     res[index--] = rNum;
                     r--;
                 }
             }else{
                 if(lNum > rNum){
                     res[index++] = rNum;
                     r--;
                 }else{
                     res[index++] = lNum;
                     l++;
                 }
             }
         }

         return res;
     }

     private int quad(int x, int a, int b, int c){
         return a*x*x + b*x + c;
     }
 }

类似Squares of a Sorted Array.