Remove Duplicates from Sorted Array

Description

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<sup>4</sup>
• -100 <= nums[i] <= 100
• nums is sorted in non-decreasing order.

Code

注意兩個解法回傳的值 j & j + 1 的不同，是因為前者 insert 完後才將 j + 1，後者是先加再 insert。

Set

same logic as Remove Element，多用了 unordered_set<int> st 來幫助我們判斷是否有 duplicate。

class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
        unordered_set<int> st;  
        int i, j;
        for(i = 0, j = 0; i < nums.size(); i++) {
            if(st.count(nums[i]) == 0) {
                nums[j++] = nums[i];
            } 
            st.insert(nums[i]);
        }
        return j;
    }
};

Two pointer

和 Remove Element 比較，作法相同，只有 i, j 的起始值不同。

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

Source

• Remove Duplicates from Sorted Array - LeetCode