### 题目

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].


### log(n2)朴素方法

public static int[] twoSum(int[] nums, int target) {
if (nums.length < 2) {
throw new IllegalArgumentException();
}
int[] result = new int[] {-1,-1};
for (int i = 0; i < nums.length-1; i++) {
for (int j = i+1; j < nums.length; j++) {
if (nums[i] + nums[j] == target) {
result[0] = i;
result[1] = j;
return result;
}
}
}
return result;
}


### log(n)解法

public static int[] twoSumLogN(int[] nums, int target) {
if (nums.length < 2) {
throw new IllegalArgumentException();
}
Map<Integer,Integer> map = new HashMap<>();
for (int i = 0; i < nums.length; i++) {
int diff = target - nums[i];
if (map.containsKey(diff)) {
return new int[] {map.get(diff),i};
}
map.put(nums[i],i);
}
return new int[] {0,0};
}