Java for LeetCode 041 First Missing Positive

標籤：

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.

解題思路一：

剛看到題目的時候感覺無從下手，後來仔細理解題意，需要找到first missing positive integer，只需要找到第一個即可，因此我們可以開一個布爾數組，儲存i是否存在，JAVA實現如下:

public int firstMissingPositive(int[] nums) {boolean[] num=new boolean[nums.length + 1];        for (int i = 0; i < nums.length; i++)             if (nums[i] > 0&&nums[i] <= nums.length)            num[nums[i]-1] = true;        for (int i = 0; i < num.length; i++)            if (!num[i])                return i+1;        return 0;    }

 

 解題思路二：使用bool數組會有O(N)的空間複雜度，直接採用交換的方法可以避免，JAVA實現如下：

static public int firstMissingPositive(int[] nums)  {       if(nums.length==0)       return 1;        for (int i = 0; i < nums.length; ) {            if (nums[i] >= 0 &&nums[i] < nums.length &&nums[i] != i && nums[i] != nums[nums[i]]){            int temp=nums[i];            nums[i]=nums[nums[i]];            nums[temp]=temp;            }            else i++;        }        for (int i = 1; i < nums.length; ++i)            if (nums[i] != i)                return i;        return nums[0] == nums.length ? nums.length + 1 : nums.length;    }

 

Java for LeetCode 041 First Missing Positive