题目

We are playing the Guess Game. The game is as follows:

I pick a number from 1 to n. You have to guess which number I picked.

Every time you guess wrong, I’ll tell you whether the number is higher or lower.

You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):

-1 : My number is lower
1 : My number is higher
0 : Congrats! You got it!

Example:

n = 10, I pick 6.
Return 6.

解题思路

二分法
注意
求中间值的时候不能使用

middle = (start + end) / 2;
因为 (start + end) 可能溢出
需要用如下公式
middle = start + (end – start)/2;

代码

// Forward declaration of guess API.
// @param num, your guess
// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
int guess(int num);

class Solution {
public:
    int guessNumber(int n) {
        if (0 == guess(n)) 
            return n;
        
        int ret, start, end, middle;
        start = 1;
        end = n;
       
        while (start < end) {
            middle = start + (end - start)/2;
            ret = guess(middle);
            printf("1 i:%d,j:%d,%d\n", start,end,middle);
            if (0 == ret) {
                return middle;
            } else if (1 == ret) {
                start = middle;
            } else {
                end = middle;
            }
            printf("2 %d\n", middle);
        }
        
        return middle;
    }
};