## Insert Delete GetRandom O(1)

### 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.

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();


### 问题分析

1. insert(val)：当val不存在时在集合中插入数据，如果成功插入了，返回true；否则返回false
2. remove(val)：当val存在时删除集合中的val，如果成功删除了，返回true；否则返回false
3. 返回集合中的一个随机数据，每一个元素必须有被获取的相同的可能性

### 代码实现

Map实现：

class RandomizedSet {
map<int,int> values;
int cur;
public:
/** Initialize your data structure here. */
RandomizedSet() {
cur=0;
}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
bool insert(int val) {
if(values.find(val) != values.end()) {
return false;
}
values[val] = cur;
cur++;
return true;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
bool remove(int val) {
if(values.find(val) != values.end()) {
map<int, int>::iterator it;
map<int, int>::iterator saveIt;
for(it=values.begin();it!=values.end();it++) {
if(it->first==val) {
saveIt = it;
break;
}
}
values.erase(it);
return true;
}
return false;
}

/** Get a random element from the set. */
int getRandom() {
int r = rand()% values.size();
map<int, int>::iterator it;
int i=0;
for(it=values.begin();it!=values.end();it++) {
if(i==r) {
return it->first;
}
i++;
}
return 0;
}
};


set实现

class RandomizedSet {
set<int> values;
public:
/** Initialize your data structure here. */
RandomizedSet() {

}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
bool insert(int val) {
if(values.find(val) != values.end()) {
return false;
}
values.insert(values.begin(), val);
return true;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
bool remove(int val) {
if(values.find(val) != values.end()) {
values.erase(val);
return true;
}
return false;
}

/** Get a random element from the set. */
int getRandom() {
int r = rand()% values.size();
set<int>::iterator it;
int i=0;
for(it=values.begin();it!=values.end();it++) {
if(i==r) {
return *it;
}
i++;
}
return 0;
}
};