g0301_0400.s0398_random_pick_index.Solution Maven / Gradle / Ivy

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Java-based LeetCode algorithm problem solutions, regularly updated

There is a newer version: 1.37

package g0301_0400.s0398_random_pick_index;

// #Medium #Hash_Table #Math #Randomized #Reservoir_Sampling
// #2022_07_15_Time_102_ms_(83.94%)_Space_81.3_MB_(76.10%)

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Random;

@SuppressWarnings("java:S2245")
public class Solution {
    // O(n) time | O(n) space
    private Map> map;
    private Random rand;

    public Solution(int[] nums) {
        map = new HashMap<>();
        rand = new Random();
        for (int i = 0; i < nums.length; i++) {
            map.computeIfAbsent(nums[i], k -> new ArrayList<>()).add(i);
        }
    }

    public int pick(int target) {
        List list = map.get(target);
        return list.get(rand.nextInt(list.size()));
    }
}