[hard]41. First Missing Positive

41. 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.

挺简单的，先把所有非正数的绝对值设为无限大

然后，从头枚举，若该数的绝对值小于vector大小，将其绝对值位的数设为负

枚举，第一个非负位的下标为答案

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        nums.push_back(-1);
        for (int i = 0; i < nums.size(); ++ i)
            if (nums[i] <= 0)
                nums[i] = nums.size();

        for (int i = 0; i < nums.size(); ++ i) {
            int x = (nums[i] > 0) ? (nums[i]) : (-nums[i]);

            if (x >= nums.size())
                continue;
            if (nums[x] > 0)
                nums[x] *= -1;
        }

        for (int i = 1; i < nums.size(); ++ i)
            if(nums[i] > 0)
                return i;

        return (int)nums.size();
    }
};