LeetCode First Missing Positive
来源:互联网 发布:王坚阿里云纷争马云 编辑:程序博客网 时间:2024/06/13 03:28
Description:
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:
一开始只想到了正常的排序,但是后来发现有一种巧妙的想法,只要我们保证最后能够实现某种排序,排完之后的结果如下[1,2,3,x,5,6,x...]那么4即为我们所求。
实现这种排序,可以用桶排序,每个nums元素保证nums[i] - 1 = i即可。
注意,对于<=0和>n的元素可以不予考虑,另外如果有两个同样的元素,那么进行一些特判。
<span style="font-size:18px;">public class Solution {public int firstMissingPositive(int[] nums) {int n = nums.length;for (int i = 0; i < n; i++) {while (nums[i] - 1 != i) {if (nums[i] <= 0 || nums[i] >= n)break;if (nums[i] == nums[nums[i] - 1])break;swap(nums, i, nums[i] - 1);}}for (int i = 0; i < n; i++)if (nums[i] != i + 1)return i + 1;return n + 1;}public void swap(int[] nums, int a, int b) {int temp = nums[a];nums[a] = nums[b];nums[b] = temp;}}</span>
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