[Leetcode]Two Sum

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9

Output: index1=1, index2=2
利用map来实现。map中的所有key都为target减去当前遍历数字后还需要的值，对应的value为当前遍历的序号。这样，通过检查数组中的每一个数是否在map的key当中，就可以直到是否存在两个数相加为target。

public int[] twoSum(int[] numbers, int target) {
int[] res = new int[2];
if (numbers == null || numbers.length == 0) {
return numbers;
}

HashMap<Integer, Integer> hm = new HashMap<Integer, Integer>();
for (int i = 0; i < numbers.length; i++) {
if (!hm.containsKey(numbers[i])) {
hm.put(target - numbers[i], i);
} else {
res[0] = hm.get(numbers[i]) + 1;
res[1] = i + 1;
break;
}
}
return res;
}