[leetcode] First Missing Positive

原创于 2015-02-01 14:21:15 发布 · 623 阅读

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本文介绍了一种算法，用于在一个未排序的整数数组中找到第一个缺失的正整数。算法运行时间复杂度为O(n)，且使用常量空间。

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First Missing Positive

Total Accepted: 27915 Total Submissions: 121776 My Submissions

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.

package com.wyt.leetcodeOJ;

import java.util.HashMap;
import java.util.Map;

public class FirstMissingPositive {

	public static void main(String[] args) {
		int[] A = {3,4,-1,1};
		System.out.println(firstMissingPositive(A));
	}

	public static int firstMissingPositive(int[] A) {
		
		if(A == null || A.length == 0) {
			return 1;
		}
		
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        
        for (int i = 0; i < A.length; i++) {
			if (!map.containsValue(A[i])) {
				map.put(A[i], A[i]);
			}
		}
        int result = 1;
        while (map.get(result)!=null) {
        	result++;
		}
        
        return result;
    }
}