1429. First Unique Number

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https://leetcode.com/problems/first-unique-number

You have a queue of integers, you need to retrieve the first unique integer in the queue.
Implement the FirstUnique class:
  • FirstUnique(int[] nums) Initializes the object with the numbers in the queue.
  • int showFirstUnique() returns the value of the first unique integer of the queue, and returns -1 if there is no such integer.
  • void add(int value) insert value to the queue.

Example 1:
Input: 
["FirstUnique","showFirstUnique","add","showFirstUnique","add","showFirstUnique","add","showFirstUnique"]
[[[2,3,5]],[],[5],[],[2],[],[3],[]]
Output: 
[null,2,null,2,null,3,null,-1]

Explanation: 
FirstUnique firstUnique = new FirstUnique([2,3,5]);
firstUnique.showFirstUnique(); // return 2
firstUnique.add(5);            // the queue is now [2,3,5,5]
firstUnique.showFirstUnique(); // return 2
firstUnique.add(2);            // the queue is now [2,3,5,5,2]
firstUnique.showFirstUnique(); // return 3
firstUnique.add(3);            // the queue is now [2,3,5,5,2,3]
firstUnique.showFirstUnique(); // return -1
Example 2:
Input: 
["FirstUnique","showFirstUnique","add","add","add","add","add","showFirstUnique"]
[[[7,7,7,7,7,7]],[],[7],[3],[3],[7],[17],[]]
Output: 
[null,-1,null,null,null,null,null,17]

Explanation: 
FirstUnique firstUnique = new FirstUnique([7,7,7,7,7,7]);
firstUnique.showFirstUnique(); // return -1
firstUnique.add(7);            // the queue is now [7,7,7,7,7,7,7]
firstUnique.add(3);            // the queue is now [7,7,7,7,7,7,7,3]
firstUnique.add(3);            // the queue is now [7,7,7,7,7,7,7,3,3]
firstUnique.add(7);            // the queue is now [7,7,7,7,7,7,7,3,3,7]
firstUnique.add(17);           // the queue is now [7,7,7,7,7,7,7,3,3,7,17]
firstUnique.showFirstUnique(); // return 17
Example 3:
Input: 
["FirstUnique","showFirstUnique","add","showFirstUnique"]
[[[809]],[],[809],[]]
Output: 
[null,809,null,-1]

Explanation: 
FirstUnique firstUnique = new FirstUnique([809]);
firstUnique.showFirstUnique(); // return 809
firstUnique.add(809);          // the queue is now [809,809]
firstUnique.showFirstUnique(); // return -1

Constraints:
  • 1 <= nums.length <= 10^5
  • 1 <= nums[i] <= 10^8
  • 1 <= value <= 10^8
  • At most 50000 calls will be made to showFirstUnique and add.
---
Space - O(N)
Time - O(N) - queue poll
---
class FirstUnique {
// Frequency of occurence of each number
Map<Integer, Integer> freqMap;
Queue<Integer> q;
public FirstUnique(int[] nums) {
q = new LinkedList<Integer>();
freqMap = new HashMap<Integer, Integer>();
for (int i = 0; i < nums.length; i++) {
add(nums[i]);
}
}
public int showFirstUnique() {
// discard numbers added later that increase freq > 1,
while (!q.isEmpty() && freqMap.get(q.peek()) > 1) {
q.poll();
}
return q.isEmpty() ? -1 : q.peek();
}
public void add(int value) {
freqMap.put(value, freqMap.getOrDefault(value, 0) + 1);
if (freqMap.get(value) == 1) {
q.offer(value);
}
}
}
/**
* Your FirstUnique object will be instantiated and called as such:
* FirstUnique obj = new FirstUnique(nums);
* int param_1 = obj.showFirstUnique();
* obj.add(value);
*/
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