442. Find All Duplicates in an Array

Problem:

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements that appear twice in this array.

Could you do it without extra space and in O(n) runtime?

Example:

Input:
[4,3,2,7,8,2,3,1]

Output:
[2,3]

思路：

Solution (C++):

vector<int> findDuplicates(vector<int>& nums) {
    if (nums.empty())  return vector<int>{};
    int n = nums.size();
    vector<int> res;
    
    for (int i = 0; i < n; ++i) {
        while (nums[i] != nums[nums[i]-1])  swap(nums[i], nums[nums[i]-1]);
    }
    for (int i = 0; i < n; ++i) {
        if (nums[i] != i+1)  res.push_back(nums[i]);
    }
    return res;
}

性能：

Runtime: 172 ms  Memory Usage: 12.5 MB

思路：

Solution (C++):

vector<int> findDuplicates(vector<int>& nums) {
    if (nums.empty())  return vector<int>{};
    int n = nums.size();
    vector<int> res;
    
    for (int i = 0; i < n; ++i) {
        nums[abs(nums[i])-1] = -nums[abs(nums[i])-1];
        if (nums[abs(nums[i])-1] > 0)  res.push_back(abs(nums[i]));
    }
    return res;
}

性能：

Runtime: 120 ms  Memory Usage: 12.7 MB