First Missing Positive

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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，该算法运行时间为O(n)，并使用常数空间。文章通过具体实例说明了如何实现这一算法，并附带了一个详细的代码示例。

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Problem:

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:

read blog :http://www.cnblogs.com/AnnieKim/archive/2013/04/21/3034631.html

very clear explanation

One thing for remark:

when swap,

int tmp = A[i];

so next,

A[i] = A[tmp-1]; A[tmp-1] = tmp;

not A[i] = A[A[i]-1]; A[A[i]-1] = tmp

Because A[i] already changed. This may cause TLE

Be careful to details


    public int firstMissingPositive(int[] A) {
       //using O(n) time: operate swap on array
       if(A.length < 1) return 1;
       int i = 0;
    
       while(i < A.length){
           if(A[i] != i+1 && A[i] >=1 && A[i] <= A.length && A[A[i]-1] != A[i]){
               // if element <0 or element > n, ignore
               //swap A[i] = A[A[i]-1]
               int tmp = A[i];
               A[i] = A[tmp-1];   // A[tmp-1], A[A[i]-1] is incorrect
               A[tmp-1] = tmp;
           }
           else i++;
       }
       int j = 0;
       for(j=0; j<A.length; j++){
           if(A[j] != j+1){
               return j+1;
 //              break;
           }
       }
       return A.length+1;
    }