【LeetCode】560.Subarray Sum Equals K(Medium)解题报告

【LeetCode】560.Subarray Sum Equals K(Medium)解题报告

题目地址：https://leetcode.com/problems/subarray-sum-equals-k/description/

题目描述：

  Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

  Example 1:

Input:nums = [1,1,1], k = 2

Output: 2

  Note:

The length of the array is in range [1, 20,000].

The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

Solution1:

//Edward Shi
//两种方法，第一种，暴力解
//time:O(n^2)
//space:O(1)
class Solution {
public int subarraySum(int[] nums, int k) {
int res = 0;
for(int i=0;i<nums.length;i++){
int sum=0;
for(int j=i;j<nums.length;j++){
sum+=nums[j];
if(sum==k){
res++;
}
}
}
return res;
}
}

Solution2:

//Edward Shi
//两种方法，第二种，two sum思想
//time:O(n)
//space:O(n)
class Solution {
public int subarraySum(int[] nums, int k) {
int res = 0;
int sum = 0;
HashMap<Integer,Integer> map = new HashMap<>();  //key是总和，value是数目
map.put(0,1);
for(int i=0 ; i<nums.length ;i++){
sum += nums[i];
if(map.containsKey(sum-k)){   //总体减前面一部分
res += map.get(sum-k);
}
map.put(sum,map.getOrDefault(sum,0)+1);  //java8新函数
}
return res;
}
}

Date：2018年1月31日