First Missing Positive

描述

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.

分析

本质上是桶排序(bucket sort)，每当 A[i]!= i+1 的时候，将A[i]与A[A[i]-1]交换，直到无法交换为止，终止条件是 A[i]== A[A[i]-1]。

代码

  1. // First Missing Positive
  2. // 时间复杂度O(n)，空间复杂度O(1)
  3. public class Solution {
  4.     public int firstMissingPositive(int[] nums) {
  5.         bucket_sort(nums);
  7.         for (int i = 0; i < nums.length; ++i)
  8.             if (nums[i] != (i + 1))
  9.                 return i + 1;
  10.         return nums.length + 1;
  11.     }
  12.     private static void bucket_sort(int[] A) {
  13.         final int n = A.length;
  14.         for (int i = 0; i < n; i++) {
  15.             while (A[i] != i + 1) {
  16.                 if (A[i] < 1 || A[i] > n || A[i] == A[A[i] - 1])
  17.                     break;
  18.                 // swap
  19.                 int tmp = A[i];
  20.                 A[i] = A[tmp - 1];
  21.                 A[tmp - 1] = tmp;
  22.             }
  23.         }
  24.     }
  25. }

相关题目

原文: https://soulmachine.gitbooks.io/algorithm-essentials/content/java/sorting/bucket-sort/first-missing-positive.html