First Missing Positive_Leetcode_#41

1 题目

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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2 解法

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

public void swap(int[] nums, int i, int j){
int temp = nums[i];
nums[i] = nums[j];
nums[j] = temp;
}
}