題目:

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 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.

給定一數n,題目會在1~n的範圍選一個目標數(target)
題目有提供函數guess(x)來判斷target和x的關係
若x>target,回傳-1
x< target,回傳1
x==target,回傳0
透過該函數推測並回傳target

這題基本上就是在和題目玩終極密碼(一個充斥二分搜要素的兒時遊戲)

# The guess API is already defined for you.
# @param num, your guess
# @return -1 if num is higher than the picked number
#          1 if num is lower than the picked number
#          otherwise return 0
# def guess(num: int) -> int:

class Solution:
    def guessNumber(self, n: int) -> int:
        l=1
        r=n
        while l<=r:
            mid=(l+r)//2
            if guess(mid)==1:
                l=mid+1
            elif guess(mid)==-1:
                r=mid-1
            else:
                return mid

在1~n的範圍進行二分搜
設下限l=1跟上限r=n,mid定義為(1+r)//2
若guess(mid)等於1(mid< target),l提高到mid+1
若guess(mid)等於-1(mid>target),r降低到mid-1
一旦guess(mid)等於0就回傳mid(猜中了)
最後執行時間31ms(faster than 91.30%)

那我們下題見