[leetcode] First Missing Positive
2015-02-07 19:31
411 查看
First Missing Positive
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.
思路:
因为是O(n)的复杂度,所以肯定不能先进行排序了。之前接触过,知道思路就是将a[0]放1,a[1]放2,如此持续下去,一直到a[i]无法满足a[i]==i+1时,i+1就是缺少的那个正数。具体做法就是判断a[i]的元素是否是一个小于n的正数,如果不是,继续a[i+1],如果是,就判断a[i]与i+1是否相等。如果相等,就继续a[i+1],如果不等,则交换a[i]与a[a[i]-1],因为a[1]=2,那么a[a[1]-1]=a[2-1]=a[1],即希望听过这样的交换使a[i]=i+1。交换之前有个条件要注意,就是a[i]与a[a[i]-1]是否相等,如果不等交换才有意义,如果相等进行交换那就是无限循环了,此时也只能继续a[i+1]了。
题解:
class Solution { public: int firstMissingPositive(int a[], int n) { int i=0; while (i<n) { if(a[i]<=0 || a[i]>n) { i++; continue; } if(a[i]!=i+1 && a[i]!=a[a[i]-1]) swap(a[i], a[a[i]-1]); else i++; } for(i=0;i<n;i++) if(a[i]!=i+1) return i+1; return n+1; } };
View Code
相关文章推荐
- leetcode -- First Missing Positive
- [LeetCode] First Missing Positive (Hasing)
- LeetCode-First Missing Positive
- LeetCode--first-missing-positive
- leetcode_c++:First Missing Positive(041)
- LeetCode 041 First Missing Positive
- 【leetcode】First Missing Positive
- LeetCode 41 First Missing Positive
- leetcode之first missing positive
- leetcode 041 First Missing Positive
- leetcode 41 First Missing Positive
- LeetCode : First Missing Positive [java]
- [LeetCode(Q41)] First Missing Positive (乱序数组中寻找第一个未出现的正整数)
- LeetCode 41 First Missing Positive(找到数组中第一个丢失的正数)
- [Leetcode] #41 First Missing Positive
- [Leetcode] First Missing Positive
- [leetcode] #41 First Missing Positive
- [LeetCode]First Missing Positive(Java)
- [LeetCode 141] First Missing Positive
- 【LeetCode 41】 First Missing Positive