Design - 380. Insert Delete GetRandom O(1)

380. Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in average O(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 the same probability of 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 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

思路：

题目要求实现一个数据结构，提供三个方法，每个方法的时间复杂度都是O(1)，插入和删除可以考虑使用map，随机返回也要求O(1), 很明显需要用数组来做，所以就是使用一个map，插入的val作为key，value保存的是这个val在数组中的下标，然后删除元素的时候，总是把从数组最后一个元素删掉，或者第一个元素删掉，这个随意，主要就是保证要删除的那个元素如果不是在最后一个就交换一下，这样每次从数组中删除某个元素的时间复杂度就是O(1)。

代码：

go：

type RandomizedSet struct {
    m map[int]int
    arr []int
}


/** Initialize your data structure here. */
func Constructor() RandomizedSet {
    return RandomizedSet{
        m : make(map[int]int),
        arr : make([]int, 0),
    }
}


/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
func (this *RandomizedSet) Insert(val int) bool {
    if _, ok := this.m[val]; ok {
        return false
    }
    
    this.m[val] = len(this.arr)
    this.arr = append(this.arr, val)
    
    return true
}


/** Removes a value from the set. Returns true if the set contained the specified element. */
func (this *RandomizedSet) Remove(val int) bool {
    index, ok := this.m[val]
    if !ok {
        return false
    }
    
    lastVal := this.arr[len(this.arr) - 1]
    if val != lastVal {
        this.arr[index] = lastVal
        this.m[lastVal] = index
    }
    // 移除最后一个元素，实现O(1)删除
    this.arr = this.arr[:len(this.arr)-1]
    delete(this.m, val)
    return true
}


/** Get a random element from the set. */
func (this *RandomizedSet) GetRandom() int {
    index := rand.Intn(len(this.arr))
    return this.arr[index]
}


/**
 * Your RandomizedSet object will be instantiated and called as such:
 * obj := Constructor();
 * param_1 := obj.Insert(val);
 * param_2 := obj.Remove(val);
 * param_3 := obj.GetRandom();
 */