Guess Number Higher or Lower
<p>We are playing the Guess Game. The game is as follows:</p>

<p>I pick a number from <code>1</code> to <code>n</code>. You have to guess which number I picked.</p>

<p>Every time you guess wrong, I will tell you whether the number I picked is higher or lower than your guess.</p>

<p>You call a pre-defined API <code>int guess(int num)</code>, which returns three possible results:</p>

<ul>
	<li><code>-1</code>: Your guess is higher than the number I picked (i.e. <code>num &gt; pick</code>).</li>
	<li><code>1</code>: Your guess is lower than the number I picked (i.e. <code>num &lt; pick</code>).</li>
	<li><code>0</code>: your guess is equal to the number I picked (i.e. <code>num == pick</code>).</li>
</ul>

<p>Return <em>the number that I picked</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 10, pick = 6
<strong>Output:</strong> 6
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 1, pick = 1
<strong>Output:</strong> 1
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> n = 2, pick = 1
<strong>Output:</strong> 1
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 2<sup>31</sup> - 1</code></li>
	<li><code>1 &lt;= pick &lt;= n</code></li>
</ul>
Binary Search; Interactive
/**
 * Forward declaration of guess API.
 * @param  num   your guess
 * @return 	     -1 if num is lower than the guess number
 *			      1 if num is higher than the guess number
 *               otherwise return 0
 * int guess(int num);
 */

class Solution {
public:
    int guessNumber(int n) {
        int left = 1, right = n;
        while (left < right) {
            int mid = left + ((right - left) >> 1);
            if (guess(mid) <= 0) {
                right = mid;
            } else {
                left = mid + 1;
            }
        }
        return left;
    }
};