LeetCode:First Missing Positive

First Missing Positive

Total Accepted: 66680 Total
Submissions: 277290 Difficulty: Hard

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.

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(M) Missing Number (H)
Find the Duplicate Number

思路：

将每个数放置到下标对应的位置。

如num=5,将它与nums[4]位置上的数交换；第二次遍历数组，第一个没与位置对应的数即为答案。

java code:

public class Solution {
public int firstMissingPositive(int[] nums) {

int n = nums.length;
for(int i=0;i<n;i++) {
while(0 < nums[i] && nums[i] <= n && nums[nums[i]-1] != nums[i])
swap(nums, nums[i]-1,i);
}
for(int i=0;i<n;i++) {
if(nums[i] != i+1) return i+1;
}
return n+1;
}

// 自定义函数：交换
private void swap(int[] nums, int i, int j) {
nums[i] ^= nums[j];
nums[j] ^= nums[i];
nums[i] ^= nums[j];
}
}