First Missing Positive

最新推荐文章于 2025-04-26 14:16:51 发布

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本文介绍了一种在未排序整数数组中寻找第一个缺失正整数的算法，该算法运行时间为O(n)，且使用常数空间。通过将每个值放置于其值-1的位置上，最后检查每个位置的值是否等于其索引+1来实现。

Question:

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.

Analysis:

Put the value to the place where the (index + 1) is equal to the value. Finally, check whether A[index] == index + 1.

public class Solution {
    public int firstMissingPositive(int[] A) {
        if (A == null || A.length == 0) return 1;
        
        for (int i = 0; i < A.length; i++) {
            while (A[i] != i + 1) {
                //A[i] == A[A[i] - 1] handles the case where there are two identical values
                // like A = [1, 1]
                if (A[i] <= 0 || A[i] >= A.length || A[i] == A[A[i] - 1]) break;
                int temp = A[i];
                A[i] = A[temp - 1];
                A[temp -1]= temp;
            }
        }
        
        for (int i = 0; i < A.length; i++) {
            if (A[i] != i + 1) {
                return i+1;
            }
        }
        return A.length + 1;        
    }
}