1.two sum

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

time:o(n) space:o(n)

最初做法用2个循环，但是用一个循环会节省一点时间，虽然时间复杂度一样。

<span style="font-size:14px;"> public int[] twoSum(int[] nums, int target) {
        HashMap<Integer,Integer> map=new HashMap<Integer,Integer>();
        int len=nums.length;
        for(int i=0;i<len;i++){
            if(map.containsKey(target-nums[i]))
                return new int[]{map.get(target-nums[i]),i};
            else
                map.put(nums[i],i);
            
        }
        
        return new int[]{-1,-1};
    }</span>