Leetcode 169 Majority Element

Given an array of size n, find the majority element. The majority element is the element that appears more than ⌊ n/2 ⌋ times.

You may assume that the array is non-empty and the majority element always exist in the array.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

 

题目解析：

我用了Hashmap的方法, 感觉不是最好的, 但是是最好想到的. 如果有更好方法的童鞋们求评论我~~么么哒~时间复杂度是O(2n), 也就是两次遍历, 但是有memory的限制, 就是开辟一个新的hash.

 

白板写的时候容易出错的就是hashmap的函数, keySet()啊, containsKey啊~之类的~

 

public int majorityElement(int[] num) {
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();

        for(int i = 0; i < num.length; i++){
            if(map.containsKey(num[i]))
                map.put(num[i], map.get(num[i]) + 1);
            else
                map.put(num[i], 1);
        }    
        for(int j: map.keySet()){
            int value = map.get(j);
            if(value > num.length / 2)
                return j;
        }
        return -1;
    }