380. Insert Delete GetRandom O(1) I & II

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

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

分析：

因为要求所有操作O(1),插入可以满足这个条件，如果用数组来存储值，getRandom可以满足O(1).但是要求不能重复和delete的时候也满足这个条件，只能使用一个map. key是值，value是位置。

public class RandomizedSet {
    ArrayList<Integer> nums;
    // value to position
    HashMap<Integer, Integer> valueToPositionMap;
    Random rand;

    public RandomizedSet() {
        nums = new ArrayList<>();
        valueToPositionMap = new HashMap<>();
        rand = new Random(System.currentTimeMillis());
    }
    
    public boolean insert(int val) {
        if (valueToPositionMap.containsKey(val)) return false;
        valueToPositionMap.put(val, nums.size());
        nums.add(val);
        return true;
    }

    public boolean remove(int val) {
        if (!valueToPositionMap.containsKey(val)) return false;
        int position = valueToPositionMap.get(val);
        if (position != nums.size() - 1) {
            // put the true last one to 'position' in nums
            int lastOne = nums.get(nums.size() - 1);
            nums.set(position, lastOne);
            valueToPositionMap.put(lastOne, position);
        }
        valueToPositionMap.remove(val);
        nums.remove(nums.size() - 1);
        return true;
    }

    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));
    }
}

第二种情况：允许有重复数值。

用set来记录同一个值出现的位置，如果被删除的数不是最后一个，则和最后一个换一下位置，然后删除就可以了。

public class RandomizedCollection {
    List<Integer> nums;
    Map<Integer, Set<Integer>> valueToPositionMap;
    Random rand;

    public RandomizedCollection() {
        nums = new ArrayList<>();
        valueToPositionMap = new HashMap<>();
        rand = new Random(System.currentTimeMillis());
    }

    public boolean insert(int val) {18         if (!isContained) {
            valueToPositionMap.put(val, new HashSet<>());
        }
        valueToPositionMap.get(val).add(nums.size());
        nums.add(val);
        return !isContained;
    }

    /**
     * Removes a value from the collection. Returns true if the collection contained
     * the specified element.
     */
    public boolean remove(int val) {
        if (!valueToPositionMap.containsKey(val)) {
            return false;
        }
        if (!valueToPositionMap.get(val).contains(nums.size() - 1)) {
            int currPos = valueToPositionMap.get(val).iterator().next();
            int lastVal = nums.get(nums.size() - 1);
            valueToPositionMap.get(lastVal).remove(nums.size() - 1);
            valueToPositionMap.get(lastVal).add(currPos);
            valueToPositionMap.get(val).remove(currPos);
            valueToPositionMap.get(val).add(nums.size() - 1);
            nums.set(currPos, lastVal);
        }
        valueToPositionMap.get(val).remove(nums.size() - 1);
        if (valueToPositionMap.get(val).isEmpty()) {
            valueToPositionMap.remove(val);
        }
        nums.remove(nums.size() - 1);
        return true;
    }

    /** Get a random element from the collection. */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));
    }
}