75. Sort Colors

Given an array 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.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note: You are not suppose to use the library’s sort function for this problem.

Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

class Solution {
    public void sortColors(int[] nums) {
    if(nums==null||nums.length<2){
        return;
    }
    int i=0;
    int j=nums.length-1;
    int index=0;
    while(index<=j){
        if(nums[index]==0){
            swap(nums,index,i);
            index++;
            i++;
        }
        //这里一定要用else if
        else if(nums[index]==2){
            swap(nums,index,j);
            j--;
        }
        else{
           index++; 
        }
    }
    }
    private void swap(int[] nums,int a,int b){
        int temp=nums[a];
        nums[a]=nums[b];
        nums[b]=temp;
    }
}