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【LeetCode】303. Range Sum Query - Immutable 解题报告(Python)

野见 2022-02-10 阅读 28


id: fuxuemingzhu

个人博客: ​​​http://fuxuemingzhu.cn/​​​


目录


  • ​​题目描述​​
  • ​​解题方法​​
  • ​​保存累积和​​
  • ​​日期​​


题目地址:​​https://leetcode.com/problems/range-sum-query-immutable/description/​​

题目描述

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.

Example:
Given nums = [-2, 0, 3, -5, 2, -1]

sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

Note:


  • You may assume that the array does not change.
  • There are many calls to sumRange function.

解题方法

保存累积和

可以直接用切片求和的方法做,也能A,但是效率太慢。

下面这个方式可以先把sums求出来,然后再调用的时候直接右边的sums-左边的sums即可得到结果。

class NumArray(object):

def __init__(self, nums):
"""
:type nums: List[int]
"""
self.sums = [0] * len(nums)
total = 0
for i, num in enumerate(nums):
total += num
self.sums[i] = total


def sumRange(self, i, j):
"""
:type i: int
:type j: int
:rtype: int
"""
if i == 0:
return self.sums[j]
else:
return self.sums[j] - self.sums[i - 1]


# Your NumArray object will be instantiated and called as such:
# obj = NumArray(nums)
# param_1 = obj.sumRange(i,j)

如果多用一个元素放在开头,那么上面的这个代码可以简化。

class NumArray(object):

def __init__(self, nums):
"""
:type nums: List[int]
"""
N = len(nums)
self.sums = [0] * (N + 1)
for i in range(1, N + 1):
self.sums[i] = self.sums[i - 1] + nums[i - 1]

def sumRange(self, i, j):
"""
:type i: int
:type j: int
:rtype: int
"""
return self.sums[j + 1] - self.sums[i]


# Your NumArray object will be instantiated and called as such:
# obj = NumArray(nums)
# param_1 = obj.sumRange(i,j)

日期

2018 年 2 月 4 日

2018 年 11 月 24 日 —— 周六快乐



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