LeetCode 41 First Missing Positive (C++, Python)

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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.

Solution C++


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class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        for (int i = 0; i < nums.size(); ++i) {
            if (nums[i] <= 0 || nums[i] > nums.size()) {
                continue;
            }
            if ((nums[i] != i + 1) && (nums[i] != nums[nums[i] - 1])) {
                swap(nums[i], nums[nums[i] - 1]);
                i --;
            }
        }
        for (int i = 0; i < nums.size(); ++i) {
            if (nums[i] != i + 1) return i + 1;
        }
        return nums.size() + 1;
    }
};
Solution Python


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class Solution:
    # @param nums, an integer[]
    # @return an integer
    def firstMissingPositive(self, nums):
        i = 0
        while True:
            if i >= len(nums): break
            if nums[i] <= 0 or nums[i] > len(nums):
                i += 1
                continue
            if nums[i] != i + 1 and nums[i] != nums[nums[i] - 1]:
                nums[nums[i] - 1], nums[i] = nums[i], nums[nums[i] - 1]
                i -= 1
            i += 1
        for i in range(len(nums)):
            if nums[i] != i + 1:
                return i + 1
        return len(nums) + 1

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