First Missing Positive

最新推荐文章于 2025-04-26 14:16:51 发布

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最新推荐文章于 2025-04-26 14:16:51 发布

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原文链接：http://www.cnblogs.com/zhengjiankang/p/3646805.html

本文介绍了一种在未排序整数数组中寻找第一个缺失正整数的算法，该算法能在O(n)时间内运行并使用常数空间。通过交换数组内的元素使每个位置上的元素值等于其索引加一，从而快速找到缺失的数值。

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Given an unsorted integer array, find the first missing positive integer.

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.
Your algorithm should run in O(n) time and uses constant space.

Solution: Although we can only use constant space, we can still exchange elements within input A!
Swap elements in A and try to make all the elements in A satisfy: A[i] == i + 1.
Pick out the first one that does not satisfy A[i] == i + 1.

 1 class Solution {
 2 public:
 3     int firstMissingPositive(int A[], int n) {
 4         int i = 0;
 5         while(i < n) {
 6             if(A[i] != i+1 && A[i] >= 1 && A[i] <= n && A[i] != A[A[i]-1]) {
 7                 swap(A[i], A[A[i]-1]);
 8             }
 9             else i++;
10         }
11         for(int i = 0; i < n; i++) {
12             if(A[i] != i+1) return i+1;
13         }
14         return n+1;
15     }
16 };

转载于:https://www.cnblogs.com/zhengjiankang/p/3646805.html