# 【Leetcode】Insert Delete GetRandom O(1)

2016-08-08 09:36:44         来源：举头天外望，无我这般人

Design a data structure that supports all following operations inaverageO(1)time.

1. `insert(val): Inserts an item val to the set if not already present.`
2. `remove(val): Removes an item val from the set if present.`
3. `getRandom: Returns a random element from current set of elements. Each element must have thesame probabilityof being returned. `

`Example:`

``````// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();
``````

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`思路：`

`O(1)的插入删除明显考虑用hash set或map，set存没法getRandom，则自然想到用hash map`

`因为要getRandom()，即key必须是连续的，当删除某元素时可能导致key不连续，所以当删除元素时要保证key空间连续，简单的，我们让最后元素填补到被删除元素的位置，使得key空间连续。`

`算法：`

``````   /** Initialize your data structure here. */
public RandomizedSet() {

}

List vals = new ArrayList();
Map val2idx = new HashMap();

/**
* Inserts a value to the set. Returns true if the set did not already
* contain the specified element.
*/
public boolean insert(int val) {
if (val2idx.containsKey(val)) {
return false;
} else {
val2idx.put(val, val2idx.size());
return true;
}
}

/**
* Removes a value from the set. Returns true if the set contained the
* specified element.
*/
public boolean remove(int val) {
if (!val2idx.containsKey(val)) {
return false;
} else {
int idx = val2idx.remove(val);
if (idx < vals.size() - 1) {// 若删除的不是链表最后的元素
// 交换末尾元素和被删除元素
vals.set(idx, vals.get(vals.size() - 1));
val2idx.put(vals.get(vals.size() - 1), idx);
}
vals.remove(vals.size() - 1);
return true;
}
}

/** Get a random element from the set. */
Random r = new Random();
public int getRandom() {
return vals.get(r.nextInt(vals.size()));
}
``````