LeetCode 380. Insert Delete GetRandom O(1)
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问题描述:
Design a data structure that supports all following operations in average 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.
// 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();
随机插入,先检查值是否存在。需要实现O(1)所以必须是Hash 来实现。删除也一样。
第三个条件随机一个数,所有的数随机到的概率是相同的。数需要被存在在Array中。
所以使用一个ArrayList来存放value,然后使用一个HashMap来存放value-index的map。
这就带来了一个新的问题,删除一个数,需要做到O(1).正常的删除需要移位,复杂度是O(n).这里有一个小技巧:将array尾部的值换到被删除value的位置上,然后删除尾部的值。
代码如下:
class RandomizedSet { private Map<Integer, Integer> valueIndexMap; private List<Integer> valueList; private Random random = new Random(); /** Initialize your data structure here. */ public RandomizedSet() { valueIndexMap = new HashMap<Integer, Integer>(); valueList = new ArrayList<Integer>(); } public boolean insert(int val) { if (valueIndexMap.containsKey(val)) { return false; } int index = valueList.size(); valueList.add(index, val); valueIndexMap.put(val, index); return true; } /** * 删除一个元素,正常的做法需要移动位置,可以固定的删除最后一位,然后将最后一位和删除的额位置调换 */ public boolean remove(int val) { if (!valueIndexMap.containsKey(val)) { return false; } Integer index = valueIndexMap.get(val); int lastIndex = valueList.size() - 1; if (index != (lastIndex)) { Integer lastValue = valueList.get(lastIndex); valueList.set(index, lastValue); valueIndexMap.put(lastValue, index);// 修改最后一位的数值 } valueList.remove(lastIndex); valueIndexMap.remove(val); return true; } /** * 下一个随机数 */ public int getRandom() { return valueList.get(random.nextInt(valueList.size())); }}
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