[LeetCode]First Missing Positive,解题报告

1. 将数组中的元素存放到HashSet中，此过程去除了重复元素
2. 从1到A.length进行遍历，遍历期间哪个int值不存在于HashSet中，则这个int值为第一个丢失的正数

public class Solution {
    public int firstMissingPositive(int[] A) {
        int miss = A.length + 1;
        
        HashSet<Integer> hashset = new HashSet<Integer>();
        for (int i = 0; i < A.length; i ++) {
            hashset.add(A[i]);
        }
        
        for (int i = 1; i <= A.length; i ++) {
            if (! hashset.contains(i)) {
                miss = i;
                break;
            }
        }
        
        return miss;
    }
}

public class Solution {
    public int firstMissingPositive(int[] A) {
        int tmp, miss = A.length + 1;

        for (int i = 0; i < A.length;) {
            if (A[i] != i + 1 && A[i] >= 1 && A[i] <= A.length && A[i] != A[A[i] - 1]) {
                tmp = A[A[i] - 1];
                A[A[i] - 1] = A[i];
                A[i] = tmp;
            } else {
                i ++;
            }
        }

        for (int i = 0; i < A.length; i++) {
            if (A[i] != i + 1) {
                miss = i + 1;
                break;
            }
        }

        return miss;
    }
}