leetcode380- Insert Delete GetRandom O(1)- medium

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();

性能这么好肯定是得空间换时间。加删都是O(1)，想到要用到Hash，但是HashSet不能random存取，所以想到用HashMap。如果要随机获得的话需要把对象聚在一起不能有分散

1.List + Map + size。amortized O(1)。Map一直记录 obj-index对。删的时候有额外小技巧，list的任意删是O(n)的，但是如果你一直都只删最后一个对象那还是O(1)的，所以你要做的就是把最后一个数和要删的互换，然后删最后的就好了。

2.两个Map + size。真的O(1)。一个Map记录obj-index对，一个Map记录index-obj对。删的时候也是需要换最后一个。（细节：remove的时候Map操作一定要先更新put再remove最后一个，反过来的话当只有一个元素的时候删就会出问题，刚删完又回来了）。

细节：几个函数：list.set(int index, Obj obj); map.remove(K key); new Random().nextInt(int bound);

1. List + Map

class RandomizedSet {
    
    private int size;
    private List<Integer> list;
    private Map<Integer, Integer> map;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<Integer>();
        map = new HashMap<Integer, Integer>();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (map.containsKey(val)) {
            return false;
        }
        list.add(size,val);
        map.put(val, size);
        size++;
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val)) {
            return false;
        }
        int idx = map.get(val);
        int temp = list.get(size - 1);
        list.set(idx, temp);
        map.put(temp, idx);
        map.remove(val);
        // 记得要切实地把最后一个东西挪掉，因为你add的时候选择直接add，那你会把东西加到垃圾后面
        list.remove(--size);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        Random rdm = new Random();
        return list.get(rdm.nextInt(size));
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */

2. Map + Map

class RandomizedSet {
    
    Map<Integer, Integer> vi;
    Map<Integer, Integer> iv;
    int size;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        vi = new HashMap<Integer, Integer>();
        iv = new HashMap<Integer, Integer>();
        size = 0;
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (vi.containsKey(val)) {
            return false;
        }
        vi.put(val, size);
        iv.put(size, val);
        size++;
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!vi.containsKey(val)) {
            return false;
        }
        int index = vi.get(val);
        int lastVal = iv.get(size - 1);
        // 千万小心这里put和remove不能换位置，为了避免只有一个条目的情况，你先删再加的话就把原来的东西加回去了。
        vi.put(lastVal, index);
        vi.remove(val);
        iv.put(index, lastVal);
        iv.remove(size - 1);
        size--;
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return iv.get(new Random().nextInt(size));
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */