0041_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.
JAVA
方法一
如果数组中不缺少数字,那么返回的结果应该是最大的那个数字+1。如果数组中缺少数字,那么缺少的数字之后的每一个数字的值,都可以看做是该数字减1的数字与1的和,例如{1,2,3,4,6,7,8}中,由于缺少5,所以6可以看做是5+1,7可以看做是6+1,8可以看做是7+1。此时需要计算出当前数组的和与预计该有的和的差值,此差值代表比缺少的的数字大1的数字有多少个,再用该数组中的最大值减去这个差值,即可的到缺少的数字。
由于没有考虑到数组中会有重复的数字,所以WA
public class Solution { public int firstMissingPositive(int[] nums) { int sum = 0; int count = 0; int result = 0; int targetSum = 0; int max = 0; for(int i = 0; i < nums.length; ++i){ if(nums[i] > 0){ sum += nums[i]; ++count; targetSum += count; if(max < nums[i]){ max = nums[i]; } } } if(sum > targetSum ){ result = max - (sum - targetSum); }else{ result = count+1; } return result; }}
方法二
还没想出来
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