find the first missing positive integer@leetcode

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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.

» Solve this problem

We need to pay attention to two constraints, 1) Constant additional space. 2)O(n) time cost. So we can't sort the array first and can't copy the array to another array. While the key point here, is to find the first missing positive integer of the array. So we can make use of the index. We iterate through the whole array, to store A[i] = i+1, if i+1 exists in the array. We do the swap.

In the swap loop, we need to be careful.
In the first, my loop is
for( int i=0; i<n; i++)
{
   if(A[i]<n&&A[i]>0)
   swap(A[i], A[A[i]-1]);
}

In this case, it will miss some elements in the array. For example, -1, 4, 2, 1, 9, 10. After first swap, A[3]=4, A[1]=1. So 1 will be missed to be swapped. So the loop here is very important.