Single Number

Given an array of integers, every element appears twice except for one. Find that single one.

Note: Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

原始代码（HashMap）

class Solution {
    public int singleNumber(int[] nums) {
        Map< Integer, Integer > hm = new HashMap< Integer, Integer >();
        int res = 0;
        for (int i : nums) {
            hm.put(i, hm.getOrDefault(i, 0) + 1);
        }
        
        for (Map.Entry< Integer, Integer > entry : hm.entrySet()) {
            if (entry.getValue() == 1) {
                res = entry.getKey();
            }
        }
        return res;
    }
}

---

• 该题的基本思路是将数组遍历后，把元素和其出现次数存入HashMap中。
• 之后再对HashMap进行遍历，找到出现次数为一的元素并返回。

优化代码

class Solution {
    public int singleNumber(int[] nums) {
        int res = nums[0];
        for (int i = 1; i < nums.length; i++) {
            res = res ^ nums[i];
        }
        return res;
    }
}

---

• 优化代码的解题思路是使用异或运算。当两个元素不相等的时候返回1，两元素相等返回0。
• A ^ A = 0, A ^ B ^ A = B