leetcode-136-只出现一次的数字

问题：

package com.example.demo;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class Test136 {

    /**
     * 借助hash表
     */
    public int singleNumber(int[] nums) {
        Set<Integer> set = new HashSet<>();
        for (int i = 0; i < nums.length; i++) {
            if (set.contains(nums[i])) {
                set.remove(nums[i]);
            } else {
                set.add(nums[i]);
            }
        }
        int res = 0;
        Iterator<Integer> iterator = set.iterator();
        while (iterator.hasNext()) {
            res = iterator.next();
        }
        return res;
    }

    /**
     * 数学方式：res = 2(a + b + c) - (a + a + b + b + c)  结果就是c，也就是要找的值
     */
    public int singleNumber2(int[] nums) {
        int sum = 0;
        Set<Integer> set = new HashSet<>();
        for (Integer integer : nums) {
            sum += integer;
            set.add(integer);
        }

        int twoSum = 0;
        for (Integer integer : set) {
            twoSum += integer;
        }
        return 2 * twoSum - sum;
    }

    /**
     * 利用位运算，相同的值异或为0，不同的值异或为1
     */
    public int singleNumber1(int[] nums) {
        int res = nums[0];
        for (int i = 1; i < nums.length; i++) {
            res ^= nums[i];
        }
        return res;
    }

    public static void main(String[] args) {
        Test136 t = new Test136();
        int[] arr = {2, 2, 1, 1, 3};
        int i = t.singleNumber2(arr);
        System.out.println(i);
    }
}