Sort Colors

题目描述：

Given an array with n objects colored red, white or blue, sort them 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.
解题思路：
类似于快排中partition过程。不过要用三个指针，一前begin，一中current，一后end,

begin与current都初始化指向数组首部，end初始化指向数组尾部。

（1）current遍历整个数组序列，current指1时，不交换，current++；

（2）current指0时，与begin交换，而后current++，begin++；

（3）current指2时，与end交换，而后current不变，end--。<
4000
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AC代码如下：

class Solution {
public:
void sortColors(vector<int>& nums) {
if (nums.size() == 0) return;
int begin = 0, current = 0, end = nums.size() - 1;
while (current <= end){
if (nums[current] == 0){
swap(nums[begin], nums[current]);
++begin;
++current;
}
else if (nums[current] == 1){
++current;
}
else{//nums[middle]=2
swap(nums[current], nums[end]);
--end;
}
}
}
};