[Leetcode] First Missing Positive

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.

 

方法比较巧，先把所有元素的值-1，重新组织元素使A[i] = i，这样只要找到第一个A[i] != i 的 i。注意要用while循环，for循环的continue是从下一次循环开始的。

 

 1 class Solution {

 2 public:

 3     int firstMissingPositive(int A[], int n) {

 4         int tmp;

 5         for (int i = 0; i < n; ++i) {

 6             A[i]--;

 7         }

 8         int i = 0;

 9         while (i < n) {

10             if (A[i] != i && A[i] >= 0 && A[i] < n && A[i] != A[A[i]]) {

11                 tmp = A[i];

12                 A[i] = A[A[i]];

13                 A[tmp] = tmp;

14             } else {

15                 i++;

16             }

17         }

18         for (int i = 0; i < n; ++i) {

19             if (A[i] != i) {

20                 return i + 1;

21             }

22         }

23         return n + 1;

24     }

25 };