2. Output one of the repeated numbers in the array

public int findRepeatNumber(int[] nums) {
        Set<Integer> dic = new HashSet<>();
        for(int num : nums){
            if(dic.contains(num)) return num;
            dic.add(num);
        }
        return -1;
    }

for(int num : nums)

• nums Is an array , It's in there int Data of type , And then define a int Variable of type num, Every cycle , From nums Take a data from the array to print
• int ： Indicates the type of collection you want to traverse
  nums： Indicates the name of the set you want to traverse
  num： Means that you traverse each element in the set It is stored in this variable

for example ： Array {2,3,4,2,0,8,6,9,10}
for the first time ：nums [2,3,4,2,0,8,6,9,10]
    dic [2]
The second time ：nums [2,3,4,2,0,8,6,9,10]
    dic [2,3]
third time ：nums [2,3,4,2,0,8,6,9,10]
    dic [2,3,4]
The fourth time ：nums [2,3,4,2,0,8,6,9,10]
    dic [2,3,4]
Output ：2