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题目链接：https://leetcode.com/problems/insert-delete-getrandom-o1/
题目：
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.
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<Integer> vals = new ArrayList<Integer>();
Map<Integer, Integer> val2idx = 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 (val2idx.containsKey(val)) {
return false;
} else {
val2idx.put(val, val2idx.size());
vals.add(val);
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()));
}
【Leetcode】Insert Delete GetRandom O(1)
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原文地址：http://blog.csdn.net/yeqiuzs/article/details/52141124
