Subarray Sum

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
Given [-3, 1, 2, -3, 4], return [0, 2] or [1, 3].

Note
There is at least one subarray that it’s sum equals to zero.

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有一个O(N)的解法。使用Map 来记录index, sum的值。当遇到两个index的sum相同时，表示从index1+1到index2是一个解。

注意：添加一个index = -1作为虚拟节点。这样我们才可以记录index1 = 0的解。

空间复杂度：O(N)

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;
    }
}