leetCode #75 Sort Colors

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这个题目超简单是不，用计数排序就搞定了~

代码如下：

class Solution {
public:
    void sortColors(vector<int>& nums) {
        if (nums.size() == 0)
        {
            return;
        }
        int count1 = 0, count0 = 0;
        for (int num : nums)
        {
            if (num == 0)
            {
                ++count0;
            }
            if (num == 1)
            {
                ++count1;
            }
        }
        int i = 0;
        for (; i < count0; ++i)
        {
            nums[i] = 0;
        }
        for (; i < count0 + count1; ++i)
        {
            nums[i] = 1;
        }
        for (; i < nums.size(); ++i)
        {
            nums[i] = 2;
        }
            
    }
};

 可是可是，这不是遍历了两遍吗？下面介绍遍历一遍就能完成的算法，三路快排来啦：

class Solution {
public:
    void sortColors(vector<int>& nums) {
        int zero = -1;  // [0..zero] == 0
        int i = 0;  // [i..two] == 1
        int two = nums.size();  // [two..nums.size()] == 2
        while (i < two)
        {
            if (nums[i] == 1)
            {
                nums[i++] = 1;
            }
            else if (nums[i] == 0)
            {
                swap(nums[i++], nums[++zero]);
            }
            else
            {
                assert(nums[i] == 2);
                swap(nums[i], nums[--two]);
            }
        }
    }
};