1. 快排

class Solution {
    public void sortColors(int[] nums) {
        quickSort(nums,0,nums.length-1);
    }

    //快排
    public void quickSort(int[] nums,int start,int end){
        if(start < end){
            int pivot = getPivot(nums,start,end);
            quickSort(nums,start,pivot-1);
            quickSort(nums,start+1,end);
        }
    }

    //返回pivot
    public int getPivot(int[] nums,int start,int end){
        int num = nums[start];
        while(start < end){
            while(start < end && nums[end] >= num)
                end--;
            nums[start] = nums[end];
            while(start < end && nums[start] <= num)
                start++;
            nums[end] = nums[start];
        }
        nums[start] = num;
        return start;
    }

}

2. 双指针将0放到前面，2放到后面

class Solution {
    public void sortColors(int[] nums) {
        int p0 = 0,p2 = nums.length-1,i = 0;
        while(i < nums.length){
            if(nums[i] == 0 && i > p0)
                swap(nums,p0++,i);
            else if(nums[i] == 2 && i < p2)
                swap(nums,p2--,i);
            else
                i++;
        }

    }

    public void swap(int[] nums,int i,int j){
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}