# My straightforward JAVA solution (double linkedlist and map)

• Use CountNode to store keys with same count into one object, I form node into a double linkedlist with descending order of count, by adopting what I learned when solving LRU, I use head node and tail node as dummies to fast locate min and max, at the same time, avoid handling corner cases when remove a node from list.
I use 2 HashMap to fast access each CountNode based on count number and key's count based on key.
The tricky part is that, you need to be very careful to update data structure for operation inc() and dec().

• for inc operation, you need to carefully handle: key existence, new count node existence, old count node existence.

• for dec operation, also key existence, key's count becomes 0 or not, new count node existence, old count node existence.
Here is the code:

``````public class AllOne {

class CountNode {
int count;
Set<String> keySet;
CountNode previous;
CountNode next;
public CountNode(int count) {
this.count = count;
keySet = new HashSet<>();
}

}

public void remove(String key) {
keySet.remove(key);
}
}

private Map<String, Integer> keyCntMap;
private Map<Integer, CountNode> cntNodeMap;
private CountNode tail;

/** Initialize your data structure here. */
public AllOne() {
keyCntMap = new HashMap<>();
cntNodeMap = new HashMap<>();
tail = new CountNode(-1);
}

/** Inserts a new key <Key> with value 1. Or increments an existing key by 1. */
public void inc(String key) {

// if key exists
if(keyCntMap.containsKey(key)) {
int count = keyCntMap.get(key) + 1;
keyCntMap.put(key, count);
CountNode oldNode = cntNodeMap.get(count - 1);

// if current count bucket exists, just update
if(cntNodeMap.containsKey(count)) {
oldNode.remove(key);

// if not, instantiate a new bucket and update
} else {
CountNode newNode = new CountNode(count);
cntNodeMap.put(count, newNode);
oldNode.remove(key);
CountNode preNode = oldNode.previous;
}

// if old bucket is empty, remove it
if(oldNode.keySet.size() == 0) {
removeNode(oldNode);
cntNodeMap.remove(oldNode.count);
}

// if key does not exist
} else {
keyCntMap.put(key, 1);

// same logic based on if bucket of count 1 exists or not
if(cntNodeMap.containsKey(1)) {
} else {
CountNode newNode = new CountNode(1);
cntNodeMap.put(1, newNode);
CountNode preNode = tail.previous;
}
}
}

/** Decrements an existing key by 1. If Key's value is 1, remove it from the data structure. */
public void dec(String key) {

// dec method changes data structure iff key exists
if(keyCntMap.containsKey(key)) {
int count = keyCntMap.get(key) - 1;
CountNode oldNode = cntNodeMap.get(count + 1);
if(count == 0) {
keyCntMap.remove(key);
oldNode.remove(key);

// the same logic
} else {
keyCntMap.put(key, count);
oldNode.remove(key);
if(cntNodeMap.containsKey(count)) {
} else {
CountNode newNode = new CountNode(count);
cntNodeMap.put(count, newNode);
CountNode preNode = oldNode.previous;
}
}
if(oldNode.keySet.size() == 0) {
removeNode(oldNode);
cntNodeMap.remove(oldNode.count);
}
}

}

/** Returns one of the keys with maximal value. */
public String getMaxKey() {
}

/** Returns one of the keys with Minimal value. */
public String getMinKey() {
return tail.previous.keySet.iterator().next();
}

private void removeNode(CountNode node) {
CountNode pre = node.previous;
CountNode next = node.next;
pre.next = next;
next.previous = pre;
node.previous = null;
node.next = null;
}

private void addNewNode(CountNode preNode, CountNode newNode, CountNode nextNode) {
preNode.next = newNode;
newNode.previous = preNode;
newNode.next = nextNode;
nextNode.previous = newNode;
}
}

/**
* Your AllOne object will be instantiated and called as such:
* AllOne obj = new AllOne();
* obj.inc(key);
* obj.dec(key);
* String param_3 = obj.getMaxKey();
* String param_4 = obj.getMinKey();
*/

``````

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