75. Sort Colors

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.

Example:

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

Follow up:

• A rather straight forward solution is a two-pass algorithm using counting sort.
  First, iterate the array counting number of 0's, 1's, and 2's, then overwrite array with total number of 0's, then 1's and followed by 2's.
• Could you come up with a one-pass algorithm using only constant space?

三色排序问题。使用 in-palce方式解决，就是 比较-替换。

解：
从头开始遍历， lt = 0，i = 0, rt = length -1,，如果i为0,与 lt 交换，i++, lt++;；如果是1，i++；如果是2 与 rt 交换，rt --。
以下为代码：

int lt = 0, i = 0, rt = nums.length - 1;
while (i <= rt) {
    if (nums[i] == 0) {
        int tmp = nums[i];
        nums[i] = nums[lt];
        nums[lt] = tmp;
        i++;
        lt++;
    } else if (nums[i] == 1)
        i++;
    else if (nums[i] == 2) {
        int tmp = nums[i];
        nums[i] = nums[rt];
        nums[rt] = tmp;
        rt--;
    }
}