LeetCode: First Missing Positive

一开始有失误，发现如果外面定义了int ret， for循环里第一个条件为int ret = 1的话，这里的ret跟外面的ret不一样。少数次过

 1 class Solution {

 2 public:

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

 4         // Start typing your C/C++ solution below

 5         // DO NOT write int main() function

 6         map<int, int> s;

 7         int maxs = 0;

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

 9             maxs = max(maxs, A[i]);

10             s[A[i]] = 1;

11         }

12         int ret = 1;

13         for (ret = 1; ret <= maxs; ret++) {

14             if (s.count(ret) == 0)  break;

15         }

16         return ret;

17     }

18 };

 上面代码有错，题目要求是constant memory，下面这段代码是正确的

 1 class Solution {

 2 public:

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

 4         // IMPORTANT: Please reset any member data you declared, as

 5         // the same Solution instance will be reused for each test case.

 6         for (int i = 0; i < n; i++) 

 7             if (A[i] != i+1) 

 8                 while (A[i] > 0 && A[i] < n && A[A[i]-1] != A[i]) swap(A[i], A[A[i]-1]);

 9         for (int i = 0; i < n; i++) 

10             if (A[i] != i+1) return i+1;

11         return n+1;

12     }

13 };

 C#:没有swap函数，注意swap的顺序

 1 public class Solution {

 2     public int FirstMissingPositive(int[] nums) {

 3         for (int i = 0; i < nums.Length; i++) {

 4             if (nums[i] != i + 1) {

 5                 while (nums[i] > 0 && nums[i] < nums.Length && nums[nums[i] - 1] != nums[i]) {

 6                     int tmp = nums[nums[i] - 1];

 7                     nums[nums[i] - 1] = nums[i];

 8                     nums[i] = tmp;

 9                 }

10             }

11         }

12         for (int i = 0; i < nums.Length; i++)

13             if (nums[i] != i + 1) return i + 1;

14         return nums.Length + 1;

15     }

16 }

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