抽屉原理 n+1个数出现在n个位置，必定有两个数产生冲突 练习题目 剑指offer13. 找出数组中重复的数字 LeetCode 41. First Missing Positive (hard) LeetCode 287. Find the Duplicate Number (medium) LeetCode 442. Find All Duplicates in an Array (medium) LeetCode 448. Find All Numbers Disappeared in an Array (easy)