# 137. Single Number II

Given a non-empty array of integers, every element appears  three  times except for one, which appears exactly once. Find that single one.

Note:

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

Example 1:

``````Input: [2,2,3,2]
Output: 3
``````

Example 2:

``````Input: [0,1,0,1,0,1,99]
Output: 99
``````

``````class Solution {
public:
int singleNumber(vector<int>& nums) {
int res = 0;
for (int i = 0; i < 32; ++i) {
int sum = 0;
for (int j = 0; j < nums.size(); ++j) {
sum += (nums[j] >> i) & 1;
}
res |= (sum % 3) << i;
}
return res;
}
};
``````

1. `ones`   代表第 ith 位只出现一次的掩码变量
2. `twos`  代表第 ith 位只出现两次次的掩码变量
3. `threes`  代表第 ith 位只出现三次的掩码变量

``````class Solution {
public:
int singleNumber(vector<int>& nums) {
int one = 0, two = 0, three = 0;
for (int i = 0; i < nums.size(); ++i) {
two |= one & nums[i];
one ^= nums[i];
three = one & two;
one &= ~three;
two &= ~three;
}
return one;
}
};
``````

00 (+) 1 = 01

01 (+) 1 = 10

10 (+) 1 = 00 ( mod 3)

b = b xor r & ~a;

a = a xor r & ~b;

``````class Solution {
public:
int singleNumber(vector<int>& nums) {
int a = 0, b = 0;
for (int i = 0; i < nums.size(); ++i) {
b = (b ^ nums[i]) & ~a;
a = (a ^ nums[i]) & ~b;
}
return b;
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/137

Single Number

Single Number III

https://leetcode.com/problems/single-number-ii/

https://leetcode.com/problems/single-number-ii/discuss/43294/Challenge-me-thx

https://leetcode.com/problems/single-number-ii/discuss/43296/An-General-Way-to-Handle-All-this-sort-of-questions.

https://leetcode.com/problems/single-number-ii/discuss/43297/Java-O(n)-easy-to-understand-solution-easily-extended-to-any-times-of-occurance

https://leetcode.com/problems/single-number-ii/discuss/43295/Detailed-explanation-and-generalization-of-the-bitwise-operation-method-for-single-numbers

LeetCode All in One 题目讲解汇总(持续更新中…)

 微信打赏 Venmo 打赏
（欢迎加入博主的知识星球，博主将及时答疑解惑，并分享刷题经验与总结，试运营期间前五十位可享受半价优惠～）

×

Help us with donation