LeetCode:First Missing Positive

Posted 2020-07-26 walker lee

First Missing Positive

Total Accepted: 66680 Total Submissions: 277290 Difficulty: Hard

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.

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 (M) Missing Number (H) Find the Duplicate Number

思路：

将每个数放置到下标对应的位置。

如num=5,将它与nums[4]位置上的数交换；第二次遍历数组，第一个没与位置对应的数即为答案。

java code:

public class Solution {
    public int firstMissingPositive(int[] nums) {
        
        int n = nums.length;
        for(int i=0;i<n;i++) {
            while(0 < nums[i] && nums[i] <= n && nums[nums[i]-1] != nums[i])
                swap(nums, nums[i]-1,i);
        }
        for(int i=0;i<n;i++) {
            if(nums[i] != i+1) return i+1;
        }
        return n+1;
    }
    
    // 自定义函数：交换
    private void swap(int[] nums, int i, int j) {
		nums[i] ^= nums[j];
		nums[j] ^= nums[i];
		nums[i] ^= nums[j];
    }
}