LeetCode题解 -- 字符串和数组(560)

Subarray Sum Equals K

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

方法一:Presum

时间复杂度:O(n^2)
空间复杂度:O(n)

    public int subarraySum(int[] nums, int k) {
        int length = nums.length;
        if(length == 0)
            return 0;
        int[] sum = new int[length + 1];
        sum[0] = 0;
        int count = 0;

        for(int i = 1;i <= length;i++){
            sum[i] = nums[i - 1] + sum[i - 1];
        }

        for(int i = 0;i < length;i++){
            for(int j = i;j < length;j++){
                if(sum[j + 1] - sum[i] == k){
                    count++;
                }
            }
        }

        return count;
    }

方法二:Presum+HashMap优化

时间复杂度:O(n)
空间复杂度:O(n)

    public int subarraySum(int[] nums, int k) {
        int length = nums.length;
        if(length == 0)
            return 0;
        Map<Integer,Integer> map = new HashMap<>();
        map.put(0,1);
        int sum = 0;
        int count = 0;

        for(int i = 0;i < length;i++){
            sum += nums[i];
            if(map.containsKey(sum - k)){
                count += map.get(sum - k);
            }
            map.put(sum,map.getOrDefault(sum,0)+1);
        }

        return count;
    }