first-missing-positive

packagecom.ytx.array;

importjava.util.Arrays;

/** 题目：first-missing-positive

 *     

 *     描述： Given an unsorted integer array, find the first missing positive integer.

                    For example,

                    Given[1,2,0]return3,

                    and[3,4,-1,1]return2.

                    Your algorithm should run in O(n) time and uses constant space.

 *

 *@authoryuantian xin

 *

 */

publicclass First_missing_positive {

       

       

       publicint firstMissingPositive(int[]A) {

             if(A== null || A.length< 1) return 1; 

       

       //把小于等于A.length的正数A[i]放到第A[i]-1个位置上 

       for (inti = 0; i < A.length;i++) { 

           while (A[i] > 0 &&A[i] <=A.length&& A[A[i] - 1] !=A[i]) { 

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

               A[A[i] - 1] = A[i]; 

               A[i] =tmp; 

            } 

        } 

         

       for (inti = 0; i < A.length;i++) { 

           if (A[i] !=i + 1) { 

               return i + 1; 

            } 

        } 

         

       return A.length+ 1; 

    }

       publicstatic void main(String[]args) {

             /*int A [] = {3,4,-1,1};*/

             intA [] = {1000,-1};

             intres;

             res= new First_missing_positive().firstMissingPositive(A);

             System.out.println(res);

       }

}