First Missing Positive

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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;    }