if every integer which exists in the range, set the position's value to negative.

then ans is which number is not neg.

```
public int firstMissingPositive(int[] nums) {
for(int i=0;i<nums.length;i++)
if(nums[i]<=0) nums[i] = Integer.MAX_VALUE;
for(int i=0;i<nums.length;i++){
int k = Math.abs(nums[i]);
if(k<=nums.length && nums[k-1]>0)
nums[k-1] = -nums[k-1];
}
for(int i=0;i<nums.length;i++)
if(nums[i]>0) return i+1;
return nums.length+1;
}
```