75. Sort Colors

Medium
Given an array nums with n objects colored red, white, or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white, and blue.
We will use the integers 0, 1, and 2 to represent the color red, white, and blue, respectively.
You must solve this problem without using the library's sort function.
Example 1:
Input: nums = [2,0,2,1,1,0]
Output:
[0,0,1,1,2,2]
Example 2:
Input: nums = [2,0,1]
Output:
[0,1,2]
Constraints:
  • n == nums.length
  • 1 <= n <= 300
  • nums[i] is either 0, 1, or 2.
Follow up: Could you come up with a one-pass algorithm using only constant extra space?

解題

因為顏色只有 0, 1, 2 三種,所以可以簡單地用一個陣列紀錄三種出現的次數,再依照 0, 1, 2 的順序把數字填入陣列。
Runtime: 0 ms, faster than 100%
Memory Usage: 2.1 MB, less than 72.27%
func sortColors(nums []int) {
count := make([]int, 3)
for _, num := range nums {
count[num]++
}
​
index := 0
for i:=0; i<count[0]; i++ {
nums[index] = 0
index++
}
​
for i:=0; i<count[1]; i++ {
nums[index] = 1
index++
}
​
for i:=0; i<count[2]; i++ {
nums[index] = 2
index++
}
}