41. First Missing Positive
Hard
Given an unsorted integer array, find the smallest missing positive integer.
Example 1:
nput: [1,2,0]
Output: 3
Example 2:
nput: [3,4,-1,1]
Output: 2
Example 3:
nput: [7,8,9,11,12]
Output: 1
Note:
Your algorithm should run in O(n) time and uses constant extra space.
感觉这题像是为了答案量身打造的一样.这种贪心算法是怎么想出来的
把每个正整数放到对应的索引上即可...
核心思想就是把正整数尽量往索引上放,没有被放的数字一定是特别大的或者是负数,那么这个索引再加上1就是我们要的答案
class Solution {
public:
int firstMissingPositive(vector<int>& nums) {
for(int i=0;i<nums.size();++i)
{
while(nums[i]>0&&nums[i]<nums.size()&&nums[nums[i]-1]!=nums[i])
swap(nums[i],nums[nums[i]-1]);
}
for(int i=0;i<nums.size();++i)
if(nums[i]!=i+1)
return i+1;
return nums.size()+1;
}
};