LeetCode 75.

class Solution {
    public void sortColors(int[] nums) {
        //[2,0,2,1,1,0] => [0,0,1,1,2,2]
        int[] cnt = new int[3];
        for (int i=0; i<nums.length; i++) {
            cnt[nums[i]]++;
        }
        int indA = 0, indB = 0;
        while (indA < 3) {
            while (cnt[indA]-- > 0) {
                nums[indB++] = indA;
            }
            indA++;
        }
    }
}