集合

import java.util.HashSet;

class Solution {
    public boolean containsDuplicate(int[] nums) {

        HashSet<Integer> set = new HashSet<>();

        for (int i = 0; i < nums.length; i++) {
            
            if (set.contains(nums[i])){
                return true;
            }
            else {
                set.add(nums[i]);
            }
        }
        
        return false;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

https://leetcode-cn.com/problems/contains-duplicate/