Force deduction brush question (1): sum of two numbers

1 Problem description

Given an array of integers nums And an integer target value target, Please find... In the array and For the target target the Two Integers , And return their array subscripts . You can assume that each input corresponds to only one answer . however , The same element in the array cannot be repeated in the answer . You can return the answers in any order .

2 Problem analysis

This problem , It is the first question of most of the force deduction questions , Including the author himself . In order to record the process of writing questions , It is hereby recorded in CSDN. For your reference .

First , Give an array , for example ,$[2,11,15,7]$, There are four elements in it , Here we need to find two Spy , We don't know what these two spies are worth , however , know , The sum of the two of them is $9$.

The task now is , Find these two Spy Hiding place ！

What shall I do? ？

These four guys line up next to each other , Come and check .
first ：$2$, the second ：$11$, Third ：$15$, The fourth one ：$7$.

（1）step 1：
$2$, The first one , First put it in a small black room ！

（2）step 2:
$11$, Second person , Check the , It and the people in the little dark room , Is the sum equal to $9$.

There is only $2$,$11+2=13$, No $9$.

ok , hold $11$ Shut up in a little dark house .

（3）step 3:
$15$, The third person , Check the , It and the people in the little dark room , Is the sum equal to $9$.

In the little dark room $2$,$11$, It doesn't add up $9$.

hold $15$ Shut up in a little dark house ！

（2）step 4:
$7$, The fourth person . Find out ！ It is similar to that in the little dark room $2$ The sum is equal to $9$！
These two men are spies ！
$2$ In position $0$,$7$ In position $3$！

3 python Realization

This is the code ：

class Solution:
    def twoSum(self, nums, target):
        n = len(nums)
        hashtable = dict()
        for i in range(n):
            if target - nums[i] in hashtable:
                return [hashtable[target - nums[i]], i]
            else:
                hashtable[nums[i]] = i
        return []

python Medium class Defines a class , be called Solution, It defines a function , be called twoSum.

This function , There are two inputs , named nums and target,nums It is a suspicious person who needs to be searched , It's an array .target Is the only known information .

You may think , Not ah , There's another. self, What is this? ？ Ha ha ha , No matter what ,python in , Functions defined in classes , The first parameter is written ,self.

Why? ？ Actually , I don't know much , Not a computer major , become a monk or nun late in life , In the future, I will write a small article to explain why I want to write self.

hashtable It's our little black house . At first, there was nothing in the little dark room , It's empty. .

hashtable = dict()

hashtable It's actually a hash table , Or hash table , adopt python Dictionary in , Realization , Specifically, why is it called " Hash ", later , Don't get tangled up ！

The following is judgment , Whether the sum of this number and the number in the little black room equals $9$!

if target - nums[i] in hashtable:

This is , Shut up in a little dark house ！

hashtable[nums[i]] = i

Okay , That's it ！

4 Running results

spyder The program written in the compiler ：

The operation results are as follows ：

Let's take a look at the running results in the buckle ：


Okay , It's done ！ Remember to like the collection .