LeetCode[397] Integer Replacement

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#leetcode #二进制

Given a positive integer n and you can do operations as follow:

If n is even, replace n with n/2.
If n is odd, you can replace n with either n + 1 or n - 1.

What is the minimum number of replacements needed for n to become 1?

Example 1:

Input:
8

Output:
3

Explanation:
8 -> 4 -> 2 -> 1

Example 2:

Input:
7

Output:
4

Explanation:
7 -> 8 -> 4 -> 2 -> 1
or
7 -> 6 -> 3 -> 2 -> 1

如果低位是11就加1，如果低位是01就减1
3和0x7fff是特殊情况（0x7fff按策略会变成0x8000，然而0x8000右移位的时候是以符号位1填补左侧的，于是循环永远不会结束）

class Solution {
public:
	int integerReplacement(int n) {
		if (n == INT_MAX) {   //special case
			return 32;        //因为0x8000右移位的时候是以符号位1填补左侧的，于是循环永远不会结束
		}
		int cnt = 0;
		while (n != 1) {
			if (n & 1) {
				if (n == 3) {    //special case
					cnt += 2;
					return cnt;
				}
				if (n & 0x02) {
					++n;
				}
				else {
					--n;
				}
			}
			else {
				n >>= 1;
			}
			++cnt;
		}
		return cnt;
	}
};