# LeetCode – Guess Number Higher or Lower (Java)

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.

**Java Solution**

This is a typical binary search problem. Here is a Java solution.

public int guessNumber(int n) { int low=1; int high=n; while(low <= high){ int mid = low+((high-low)/2); int result = guess(mid); if(result==0){ return mid; }else if(result==1){ low = mid+1; }else{ high=mid-1; } } return -1; } |

**What we learn from this problem? **

low+(high-low)/2 yields the same value with (low+high)/2. However, the first expression is less expensive. In addition, the following expression can be used:

low+((high-low)>>1) (low+high)>>>1

Under the assumption that high and low are both non-negative, we know for sure that the upper-most bit (the sign-bit) is zero.

So both high and low are in fact 31-bit integers.

high = 0100 0000 0000 0000 0000 0000 0000 0000 = 1073741824 low = 0100 0000 0000 0000 0000 0000 0000 0000 = 1073741824

When you add them together they may "spill" over into the top-bit.

high + low = 1000 0000 0000 0000 0000 0000 0000 0000 = 2147483648 as unsigned 32-bit integer = -2147483648 as signed 32-bit integer (high + low) / 2 = 1100 0000 0000 0000 0000 0000 0000 0000 = -1073741824 (high + low) >>> 1 = 0100 0000 0000 0000 0000 0000 0000 0000 = 1073741824

<pre><code> String foo = "bar"; </code></pre>

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