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++
    }
}