374. Guess Number Higher or Lower 
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Problem
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 will tell you whether the number I picked is higher or lower than your guess.
You call a pre-defined API int guess(int num), which returns three possible results:
-1: Your guess is higher than the number I picked (i.e.num > pick).1: Your guess is lower than the number I picked (i.e.num < pick).0: your guess is equal to the number I picked (i.e.num == pick).
Return the number that I picked.
Example 1:
Input: n = 10, pick = 6 Output: 6
Example 2:
Input: n = 1, pick = 1 Output: 1
Example 3:
Input: n = 2, pick = 1 Output: 1
Constraints:
1 <= n <= 231 - 11 <= pick <= n