LeetCode 41: First Missing Positive Solution

Master LeetCode problem 41 (First Missing Positive), a hard challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

41. First Missing Positive

Hard

Array Hash Table

Problem Explanation

Explanation

To solve this problem in O(n) time and O(1) auxiliary space, we can utilize the array itself to keep track of the missing positive integers. We iterate through the array and place each element at its correct position (i.e., nums[i] should be placed at index nums[i] - 1). After the rearrangement, we then iterate through the array again to find the first index where the value does not match the index (indicating a missing positive integer). If no such index is found, then the missing integer is one more than the length of the array.

Time Complexity: O(n)
Space Complexity: O(1)

Solution Code

class Solution {
    public int firstMissingPositive(int[] nums) {
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            while (nums[i] > 0 && nums[i] <= n && nums[nums[i] - 1] != nums[i]) {
                int temp = nums[nums[i] - 1];
                nums[nums[i] - 1] = nums[i];
                nums[i] = temp;
            }
        }
        
        for (int i = 0; i < n; i++) {
            if (nums[i] != i + 1) {
                return i + 1;
            }
        }
        
        return n + 1;
    }
}

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