Earliest Uniques in a Stream - Amazon Top Interview Questions

Problem Statement :

```Implement a data structure with the following methods:

EarliestUnique(int[] nums) constructs a new instance with the given list of numbers.
firstUnique() returns the first unique number. If there's no unique number, return -1.
Constraints

n ≤ 100,000 where n is the number of calls to add and firstUnique.

Example 1

Input

arguments = [[[1, 2, 3]], [1], [], [2], []]`

Output

[None, None, 2, None, 3]

Explanation

e = EarliestUnique([1, 2, 3])
e.earliestUnique() == 2
e.earliestUnique() == 3```

Solution :

```                        ```Solution in C++ :

class EarliestUnique {
deque<int> a;
unordered_map<int, int> c;

public:
EarliestUnique(vector<int>& nums) {
for (int x : nums) add(x);
}

a.push_back(x);
++c[x];
}

int earliestUnique() {
while (!a.empty() && c[a.front()] > 1) a.pop_front();
return a.empty() ? -1 : a.front();
}
};```
```

```                        ```Solution in Java :

import java.util.*;

class EarliestUnique {
private Map<Integer, Integer> map;
public EarliestUnique(int[] nums) {
for (int num : nums) map.put(num, map.getOrDefault(num, 0) + 1);
}

map.put(num, map.getOrDefault(num, 0) + 1);
}

public int earliestUnique() {
for (int key : map.keySet()) {
if (map.get(key) == 1)
return key;
}
return -1;
}
}```
```

```                        ```Solution in Python :

class EarliestUnique:
def __init__(self, nums):
self.counter = Counter(nums)
self.queue = deque(nums)
self._balance()

def _balance(self):
while self.queue and self.counter[self.queue[0]] > 1:
self.queue.popleft()

self.queue.append(num)
self.counter[num] += 1
self._balance()

def earliestUnique(self):
return self.queue[0] if self.queue else -1```
```

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