leetcode 001

1 Two Sum

**Difficulty: Easy **

The Link:

• https://leetcode.com/problems/two-sum/description/

Description :

Given an array of integers, return indices(索引) 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].

Solutions

Solution A:

暴力解法 two-loop runtime:7496ms,

class Solution:
  def twoSum(self,num,target):
    """
    :type nums:List[int]
    :type targrt: int
    :type: Link[int]
    """
    for i in range(len(nums)):
      for j in range(i+1,len(nums)):
        if nums[i] + nums[j] == target
          return [i,j]
    return []

Solution B:

By dict to improve the effiency of the programs.Runtime:34ms

data :       2     7   11   12   17
i    :       0     1
target-num:  7     2
look_up:    {}   {2:0}
result:     no   {1,0}

class Solution:
  def twoSum(self,num,target):
    """
    :type nums:List[int]
    :type targrt: int
    :type: Link[int]
    """
    lookup = {}
    for i in range(len(nums)):
            for j in range(i+1, len(nums)):
                if nums[i] + nums[j] == target:
                    return [i, j]
        return []