题目

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.

思路一

先找到最大值，建立 hashtable 记录是否存在。

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(n<=0)
            return 1;
        int max = 0;
        for(int i=0;i<n;i++)
        {
            if(A[i]>max)
                max = A[i];
        }
        vector<bool> table(max+1,false);
        for(int i=0;i<n;i++)
        {
            if(A[i]>0)
                table[A[i]]=true;
        }
        int i=1;
        while(i<=max)
        {
            if(table[i]==false)
                return i;
            i++;
        }
        if(i>max)
            return max+1;
        
    }
};

优化代码

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(n<=0)
            return 1;
        vector<bool> table(n+1,false);
        for(int i=0;i<n;i++)
        {
            if(A[i]>0 && A[i]<=n)
                table[A[i]]=true;
        }
        int i=1;
        while(i<=n)
        {
            if(table[i]==false)
                return i;
            i++;
        }
        return n+1;        
    }
};

思路二： O(N) time  and  O(1) space

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(n<=0)
            return 1;
        int i=0;
        while(i<n)
        {
            if(A[i]>0 && A[i]<n  && A[i]-1!=i && A[A[i]-1]!=A[i])
            {
                int tmp = A[A[i]-1];
                A[A[i]-1] = A[i];
                A[i] = tmp;
            }
            else
                i++;
        }
        int j=0;
        while(j<n && A[j]-1==j)
            j++;
        if(j<n)
            return j+1;
        return n+1;       
        
    }
};

最新 java

public class Solution {
    public int firstMissingPositive(int[] nums) {
        if(nums == null || nums.length == 0){
            return 1;
        }
        Set<Integer> hashset = new HashSet<>();
        for(int i=0; i<nums.length; i++){
            hashset.add(nums[i]);
        }
        int index = 1;
        while(hashset.contains(index)) {
            index++;
        }
        return index;
    }
}