[LeetCode]First Missing Positive

First Missing Positive

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Question

Total Accepted: 51712 Total
Submissions: 224707 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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这个问题做了好久，好多次提交都没有成功，一开始我的想法是通过加一个数减一个数的方式来做，首先是计算有多少个正数，从而判断他们的和是多少，然后减去数组中已有的，后来发现这种方法没法处理有重复值得情况。之后想到的是，利用数组下标，将数值放到对应的下标上，最后AC。

public class Solution {
public int firstMissingPositive(int[] nums) {
if(nums.length<1)return 1;

int result = 0;
for(int i=0;i<nums.length;i++){
if(nums[i]>0&&nums[i]!=i+1){
if(nums[i]>nums.length){
continue;
}
while(nums[i]!=i+1&&nums[i]>0){
if(nums[i]-1>=nums.length||nums[nums[i]-1]==nums[i])break;
int temp = nums[nums[i]-1];
nums[nums[i]-1]=nums[i];
nums[i]=temp;
}
}
}
for(int i=0;i<nums.length;i++){
if(nums[i]!=i+1){
return i+1;
}
}
return nums.length+1;
}
}