LeetCode----First Missing Positive
2014-02-12 21:33
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Given an unsorted integer array, find the first missing positive integer.
For example,
Given
and
Your algorithm should run in O(n) time and uses constant space.
分析:
思路1: 先排序,然后扫描一遍数组即可。。O(nlgn)
思路2: 总共 n 个数,对于那些大于 n 的数一定不会是我们的 first missing positive integer.
相当于我们有 n 个桶,每个桶应该从1开始到 n 放入数,第一个不满足的桶就是我们的结果。
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.
分析:
思路1: 先排序,然后扫描一遍数组即可。。O(nlgn)
思路2: 总共 n 个数,对于那些大于 n 的数一定不会是我们的 first missing positive integer.
相当于我们有 n 个桶,每个桶应该从1开始到 n 放入数,第一个不满足的桶就是我们的结果。
class Solution { public: int firstMissingPositive(int A[], int n) { bucket_sort(A, n); for(int i=0; i<n; ++i) if(A[i] != i+1) return i+1; return n+1; } private: void bucket_sort(int A[], int n){ for(int i=0; i<n; ++i){ while(A[i] != i+1){ if(A[i] <= 0 || A[i] > n || A[i] == A[A[i]-1]) break; swap(A[i], A[A[i]-1]); } } } };
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