leetcode First Missing Positive

原创于 2013-02-24 09:45:15 发布 · 861 阅读

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本文介绍了一种算法，用于在未排序的整数数组中找到第一个缺失的正整数，时间复杂度为O(n)，空间复杂度为常数。通过在原地操作数组，算法确保了效率和内存使用效率。

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.

typical 1-n consecutive number problem. only thing to note is the edge cases like [1,2]. so to do in place, store A[0]=1,A[1]=2, etc

public class Solution {
    public int firstMissingPositive(int[] A) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(A.length==0||A==null) return 1;
        for(int i=0;i<A.length;i++){
            int t=A[i];
            while(t<A.length+1&&t!=i+1&&t>0&&A[t-1]!=t){
                int tmp=A[t-1];
                A[t-1]=t;
                t=tmp;
            }
            A[i]=t;
        }
        for(int i=0;i<A.length;i++)
        if(A[i]!=i+1) return i+1;
        return A.length+1;
    }
}