LeetCode137. Single Number II

一、题目

Given an integer array nums where every element appears three times except for one, which appears exactly once. Find the single element and return it.

You must implement a solution with a linear runtime complexity and use only constant extra space.

Example 1:

Input: nums = [2,2,3,2]
Output: 3
Example 2:

Input: nums = [0,1,0,1,0,1,99]
Output: 99

Constraints:

1 <= nums.length <= 3 * 104
-231 <= nums[i] <= 231 - 1
Each element in nums appears exactly three times except for one element which appears once.

二、题解

class Solution {
public:
    int singleNumber(vector<int>& nums) {
        int n = nums.size();
        //记录所有数每一位上为1的总数
        vector<int> cnt(32,0);
        for(auto x:nums){
            for(int i = 0;i < 32;i++){
                cnt[i] += (x >> i) & 1;
            }
        }
        int res = 0;
        for(int i = 0;i < 32;i++){
        	//不是3的倍数，说明该数在这一位上为1
            if(cnt[i] % 3 != 0) res |= 1 << i;
        }
        return res;
    }
};