First Missing Positive

Leetcode Problem 41: First Missing Positive

Problem Summary

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.

Techniques

• Cycle sort algorithm
• In-place rearrangement of elements

Solution

function firstMissingPositive(nums: number[]): number {
    /// https://en.wikipedia.org/wiki/Cycle_sort
    for (let i = 0; i < nums.length; i++) {
        while (nums[i] > 0 && nums[i] <= nums.length && nums[i] !== nums[nums[i] - 1]) {
            let temp = nums[nums[i] - 1];
            nums[nums[i] - 1] = nums[i];
            nums[i] = temp;
        }
    }

    for (let i = 0; i < nums.length; i++) {
        if (nums[i] !== i + 1) {
            return i + 1
        }
    }

    return nums.length + 1
};

console.log(firstMissingPositive([7,8,9,11,12]))
console.log(firstMissingPositive([3,4,-1,1]))