LeetCode-41. First Missing Positive

Given an unsorted integer array, find the smallest missing positive integer.

Example 1:

Input: [1,2,0]
Output: 3

Example 2:

Input: [3,4,-1,1]
Output: 2

Example 3:

Input: [7,8,9,11,12]
Output: 1

Note:

Your algorithm should run in O(n) time and uses constant extra space.

题解：

把每个nums[i]放在nums[nums[i] - 1]位置上，比如1放在nums[0], 2放在nums[1]中，类推，然后找到第一个num[i]不等于i + 1的值，i + 1即为缺失值，若都存在，那么n+1为第一个缺失的，注意处理0值和大于数组长度的值。

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        if (nums.empty() == true) {
            return 1;
        }
        int n = nums.size();
        for (int i = 0; i < n; i++) {
            while (nums[i] > 0 && nums[i] < n && nums[nums[i] - 1] != nums[i]) {
                swap(nums[i], nums[nums[i] - 1]);
            }
        }
        for (int i = 0; i < n; i++) {
            if (nums[i] != i + 1) {
                return i + 1;
            }
        }
        return n + 1;
    }
};