g0001_0100.s0041_first_missing_positive.Solution Maven / Gradle / Ivy

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package g0001_0100.s0041_first_missing_positive;

// #Hard #Top_100_Liked_Questions #Top_Interview_Questions #Array #Hash_Table #Udemy_Arrays
// #2022_06_16_Time_5_ms_(50.34%)_Space_126.8_MB_(61.39%)

/**
 * 41 - First Missing Positive\.
 *
 * Hard
 *
 * Given an unsorted integer array `nums`, return the smallest missing positive integer.
 *
 * You must implement an algorithm that runs in `O(n)` time and uses constant extra space.
 *
 * **Example 1:**
 *
 * **Input:** nums = [1,2,0]
 *
 * **Output:** 3 
 *
 * **Example 2:**
 *
 * **Input:** nums = [3,4,-1,1]
 *
 * **Output:** 2 
 *
 * **Example 3:**
 *
 * **Input:** nums = [7,8,9,11,12]
 *
 * **Output:** 1 
 *
 * **Constraints:**
 *
 * *   1 <= nums.length <= 5 * 10⁵
 * *   -2³¹ <= nums[i] <= 2³¹ - 1
**/
public class Solution {
    public int firstMissingPositive(int[] nums) {
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] <= 0 || nums[i] > nums.length || nums[i] == i + 1) {
                continue;
            }
            dfs(nums, nums[i]);
        }
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != i + 1) {
                return i + 1;
            }
        }
        return nums.length + 1;
    }

    private void dfs(int[] nums, int val) {
        if (val <= 0 || val > nums.length || val == nums[val - 1]) {
            return;
        }
        int temp = nums[val - 1];
        nums[val - 1] = val;
        dfs(nums, temp);
    }
}