[LeetCode] 26. Remove Duplicates from Sorted Array 删除有序数组中的重复项

Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same. Then return the number of unique elements in nums.

Consider the number of unique elements of nums to be k, to get accepted, you need to do the following things:

Change the array nums such that the first k elements of nums contain the unique elements in the order they were present in nums initially. The remaining elements of nums are not important as well as the size of nums.
Return k.

Custom Judge:

The judge will test your solution with the following code:

int[] nums = [...]; // Input array
int[] expectedNums = [...]; // The expected answer with correct length
int k = removeDuplicates(nums); // Calls your implementation
assert k == expectedNums.length;

for (int i = 0; i < k; i++) {

assert nums[i] == expectedNums[i];

}

If all assertions pass, then your solution will be accepted.

Example 1:

Input: nums = [1,1,2]
Output: 2, nums = [1,2,_]
Explanation: Your function should return k = 2, with the first two elements of nums being 1 and 2 respectively.
It does not matter what you leave beyond the returned k (hence they are underscores).

Example 2:

Input: nums = [0,0,1,1,1,2,2,3,3,4]
Output: 5, nums = [0,1,2,3,4,_,_,_,_,_]
Explanation: Your function should return k = 5, with the first five elements of nums being 0, 1, 2, 3, and 4 respectively.
It does not matter what you leave beyond the returned k (hence they are underscores).

Constraints:

1 <= nums.length <= 3 * 10^4
-100 <= nums[i] <= 100
nums is sorted in non-decreasing order.

这道题要我们从有序数组中去除重复项，和之前那道 Remove Duplicates from Sorted List 的题很类似，但是要简单一些，因为毕竟数组的值可以通过下标直接访问，而链表不行。那么这道题的解题思路是使用快慢指针来记录遍历的坐标，最开始时两个指针都指向第一个数字，如果两个指针指的数字相同，则快指针向前走一步，如果不同，则两个指针都向前走一步，这样当快指针走完整个数组后，慢指针当前的坐标加1就是数组中不同数字的个数，代码如下：

解法一：

class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
        int pre = 0, cur = 0, n = nums.size();
        while (cur < n) {
            if (nums[pre] == nums[cur]) ++cur;
            else nums[++pre] = nums[cur++];
        }
        return nums.empty() ? 0 : (pre + 1);
    }
};

我们也可以用 for 循环来写，这里的j就是上面解法中的 pre，i就是 cur，所以本质上都是一样的，参见代码如下：

解法二：

class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
        int j = 0, n = nums.size();
        for (int i = 0; i < n; ++i) {
            if (nums[i] != nums[j]) nums[++j] = nums[i];
        }
        return nums.empty() ? 0 : (j + 1);
    }
};

这里也可以换一种写法，用变量i表示当前覆盖到到位置，由于不能有重复数字，则只需要用当前数字 num 跟上一个覆盖到到数字 nums[i-1] 做个比较，只要 num 大，则一定不会有重复（前提是数组必须有序），参见代码如下：

解法三：

class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
        int i = 0;
        for (int num : nums) {
            if (i < 1 || num > nums[i - 1]) {
                nums[i++] = num;
            }
        }
        return i;
    }
};