add leetcode problem-cn part2

算法题（国内版）/problem (English)/消失的数字(English) [missing-number-lcci].html

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+<p>An array&nbsp;contains all the integers from 0 to n, except for one number which is missing.&nbsp; Write code to find the missing integer. Can you do it in O(n) time?</p>
+
+<p><strong>Note: </strong>This problem is slightly different from the original one the book.</p>
+
+<p><strong>Example 1: </strong></p>
+
+<pre>
+<strong>Input: </strong>[3,0,1]
+<strong>Output: </strong>2</pre>
+
+<p>&nbsp;</p>
+
+<p><strong>Example 2: </strong></p>
+
+<pre>
+<strong>Input: </strong>[9,6,4,2,3,5,7,0,1]
+<strong>Output: </strong>8
+</pre>