41. First Missing Positive

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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.

解法1：

归位的思想。把每一个数字放到他该在的位置上。
C++

1

Java

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public class Solution {
    public int firstMissingPositive(int[] nums) {
        if (nums == null || nums.length == 0) {
            return 1;
        }
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            while (nums[i] <= n && nums[i] > 0 && nums[i] != nums[nums[i] - 1]) {
                int temp = nums[nums[i] - 1];
                nums[nums[i] - 1] = nums[i];
                nums[i] = temp;
            }
        }
        for (int i = 0; i < n; i++) {
            if (nums[i] != i + 1) return i + 1;
        }
        return n + 1;
    }
}