1.Two Sum

1.Two Sum

Thinking:

• Method1: Search We need to find if (target - current) is in the set, if not, add current to a hashSet for checking. Since to need to return the index according to value, so the key is value of number and value is index of number in array.

    class Solution {
        public int[] twoSum(int[] nums, int target) {
            Map<Integer, Integer> map = new HashMap<>();
            int numLength = nums.length;
            for(int i = 0; i < numLength; i++){
                if(!map.containsKey(target - nums[i])){
                    map.put(nums[i], i);
                }else{
                    return new int[]{i, map.get(target - nums[i])};
                }
            }
            return null;
        }
    }
  

二刷

1. 二刷的时候仍然想到用hash表解决该问题。键存的是数字的值，值存的是index。
2. 在遍历的时候，我们同时检查target - nums[i]是否存在。
   • 如果不存在，我们将数值加入哈希表。
   • 如果存在，我们直接返回index。

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for(int i = 0; i < nums.length; i++){
            if(!map.containsKey(target - nums[i]))
                map.put(nums[i], i);
            else
                return new int[]{i, map.get(target - nums[i])};
        }
        return null;
    }
}