[LeetCode41] 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.

Analysis:

桶排序，每次当A[i]!= i的时候，将A[i]与A[A[i]]交换，直到无法交换位置。终止条件是 A[i]== A[A[i]]。
然后i -> 0 到n走一遍就好了。

Java

public int firstMissingPositive(int[] A) {for(int i=0;i<A.length;i++){while(A[i]!=i+1){if(A[i]<=0 ||A[i]>=A.length|| A[i]==A[A[i]-1]) break;int temp = A[i];A[i] = A[A[i]-1];A[temp-1] = temp;}}for(int i=0;i<A.length;i++){if(A[i]!=i+1)return i+1;}return A.length+1;}

c++

int firstMissingPositive(int A[], int n) {int i=0;while(i<n){if(A[i]!=i+1 && A[i]>=1 &&A[i]<=n && A[A[i]-1] !=A[i])swap(A[i],A[A[i]-1]);elsei++;}for(int i=0; i<n;i++){if(A[i]!=i+1)return i+1;}return n+1;}