Subarray Sum

Description

 

Given an integer array, find a subarray where the sum of numbers is zero. Your code should return the index of the first number and the index of the last number.

Example

Example 1:

Input:  [-3, 1, 2, -3, 4]
Output: [0, 2] or [1, 3].
Explanation: return anyone that the sum is 0.

Example 2:

Input:  [-3, 1, -4, 2, -3, 4]
Output: [1,5]	
思路：某一段[l, r]的和为0， 则其对应presum[l-1] = presum[r].
presum 为数组前缀和。只要保存每个前缀和，找是否有相同的前缀和即可。

public class Solution {
    /**
     * @param nums: A list of integers
     * @return: A list of integers includes the index of the first number
     *          and the index of the last number
     */
    public ArrayList<Integer> subarraySum(int[] nums) {
        // write your code here
       
        int len = nums.length;
       
        ArrayList<Integer> ans = new ArrayList<Integer>();
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
       
        map.put(0, -1);
       
        int sum = 0;
        for (int i = 0; i < len; i++) {
            sum += nums[i];
           
            if (map.containsKey(sum)) {
                ans.add(map.get(sum) + 1);
                ans.add(i);
                return ans;
            }
            
            map.put(sum, i);
        }
       
        return ans;
    }
}