#Leetcode# 1009. Complement of Base 10 Integer

https://leetcode.com/problems/complement-of-base-10-integer/

Every non-negative integer N has a binary representation.  For example, 5 can be represented as "101" in binary, 11 as "1011" in binary, and so on.  Note that except for N = 0, there are no leading zeroes in any binary representation.

The complement of a binary representation is the number in binary you get when changing every 1 to a 0 and 0 to a 1.  For example, the complement of "101" in binary is "010" in binary.

For a given number N in base-10, return the complement of it's binary representation as a base-10 integer.

Example 1:

Input: 5
Output: 2
Explanation: 5 is "101" in binary, with complement "010" in binary, which is 2 in base-10.

Example 2:

Input: 7
Output: 0
Explanation: 7 is "111" in binary, with complement "000" in binary, which is 0 in base-10.

Example 3:

Input: 10
Output: 5
Explanation: 10 is "1010" in binary, with complement "0101" in binary, which is 5 in base-10.

代码：

class Solution {
public:
    int bitwiseComplement(int N) {
        if(N == 0) return 1;
        vector<int> ans;
        while(N) {
            ans.push_back(!(N % 2));
            N /= 2;
        }
        int sum = 0;
        for(int i = 0; i < ans.size() / 2; i ++)
            swap(ans[i], ans[ans.size() - i - 1]);
        for(int i = 0; i < ans.size(); i ++)
            sum += ans[i] * pow(2, ans.size() - 1 - i);

        return sum;
    }
    int pow(int a, int b) {
        int ans = 1;
        while(b) {
            if(b % 2) {
                ans *= a;
                b --;
            } else {
                a *= a;
                b /= 2;
            }
        }
        return ans;
    }
};

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