[Leetcode] First missing positive

First missing positive

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 shold run inO(N) Time and uses constant space.

Algorithm analysis:

Set ~ The number of A. length is placed at the corresponding position of the subscript. Other plural values or values greater than a. length are not processed.

Then traverse a again and find the first default integer.

[Note] processing of repeated numbers and negative numbers; I should be placed in the I-1 position, but if I-1 is also placed in the I-1 position, skip; if it is not a positive number, skip; skip if the number is too large;

The Code is as follows:

 1 public class Solution { 2     public int firstMissingPositive(int[] a) { 3         if(a == null || a.length == 0) return 1; 4         int length = a.length; 5         for(int i = 0; i < length; i++){ 6             if(a[i] > length - 1 || a[i] <= 0 || a[i] == a[a[i] - 1]) continue; 7             int t = a[a[i] - 1]; 8             a[a[i] - 1] = a[i]; 9             a[i] = t;10             i--;11         }12         int positive = 1;13         for(int i = 0; i < length; i++){14             if(a[i] <= 0) continue;15             if(a[i] == positive){16                 positive++;17             }else{18                 return positive;19             }20         }21         return positive;22     }23 }

This question is very good.