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 i=0; i<n;) {
if (A[i] != i+1 && A[i] > 0 && A[i] <= n && A[i] != A[A[i]-1])
swap(A[i], A[A[i]-1]);
else
++i;
}
for (int i=0; i<n; i++) {
if (A[i] != i+1)
return i+1;
}
return n+1;
}
};
1. 将資料交換到自己的位置
2. 略過那些不存在合法位置的值,比如負數,或者超過n的數
3.持續交換,直到目标的位置已經存在目标值。或者第2種情況。