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HashTable-380-Insert Delete GetRandom O(1)

2018-01-24 19:35 232 查看
Description:

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 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();


Solution:

public class RandomizedSet {
ArrayList<Integer> nums;
HashMap<Integer, Integer> locs;
java.util.Random rand = new java.util.Random();
/** Initialize your data structure here. */
public RandomizedSet() {
nums = new ArrayList<Integer>();
locs = 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) {
boolean contain = locs.containsKey(val);
if ( contain ) return false;
locs.put( val, nums.size());
nums.add(val);
return true;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
public boolean remove(int val) {
boolean contain = locs.containsKey(val);
if ( ! contain ) return false;
int loc = locs.get(val);
if (loc < nums.size() - 1 ) { // not the last one than swap the last one with this val
int lastone = nums.get(nums.size() - 1 );
nums.set( loc , lastone );
locs.put(lastone, loc);
}
locs.remove(val);
nums.remove(nums.size() - 1);
return true;
}

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