1.两数之和

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import java.util.HashMap;
import java.util.Map;

class Solution {
    public int[] twoSum(int[] nums, int target) {
       Map<Integer,Integer> map = new HashMap<>();

       for(int i=0;i<nums.length;i++){
        int complement = target-nums[i];
        if(map.containsKey(complement)){
            return new int[] {map.get(complement),i};
        }
        map.put(nums[i],i);
       }

       return new int[0];
    }
}