LEETCODE: First Missing Positive

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#LEETCODE #算法 #数组

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此算法在给定无序整数数组中找到首个缺失的正整数，时间复杂度为O(n)，且使用常量空间。通过迭代数组并交换元素到其正确位置来实现。

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.

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        for(int ii = 0; ii < n; ii ++) {
            // Use while to make number on position ii to be ii + 1 or invalid number.
            // And swap may be called more than one time.
            while(A[ii] != ii + 1 && A[ii] >= 1 && A[ii] <= n && A[ii] != A[A[ii] - 1])
                swap(A[ii], A[A[ii] - 1]); 
        }
        
        int ii = 0;
        for(; ii < n; ii ++) {
            if(A[ii] != ii + 1) return ii + 1;
        }
        return n + 1;
    }
};