0307._Range_Sum_Query_-_Mutable.md

307. Range Sum Query - Mutable

难度: Medium

刷题内容

原题连接

• https://leetcode.com/problems/range-sum-query-mutable/

内容描述

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

The update(i, val) function modifies nums by updating the element at index i to val.

Example:

Given nums = [1, 3, 5]

sumRange(0, 2) -> 9
update(1, 2)
sumRange(0, 2) -> 8
Note:

The array is only modifiable by the update function.
You may assume the number of calls to update and sumRange function is distributed evenly.

解题方案

思路 1 - 时间复杂度: update O(lgN) + sumRange O(lgN)- 空间复杂度: O(N)******

直接segment tree

class NumArray:
    def __init__(self, nums):
        """
        :type nums: List[int]
        """
        self.n = len(nums)
        self.tree = [0] * (2 * self.n)
        def buildTree(nums):
            self.tree[self.n:] = nums[:]
            for i in range(self.n-1, 0, -1):
                self.tree[i] = self.tree[2 * i] + self.tree[2 * i + 1]
        buildTree(nums)

    def update(self, i, val):
        """
        :type i: int
        :type val: int
        :rtype: void
        """
        i += self.n
        self.tree[i] = val
        while i > 0:
            self.tree[i // 2] = self.tree[i // 2 * 2] + self.tree[i // 2 * 2 + 1]
            i //= 2

    def sumRange(self, i, j):
        """
        :type i: int
        :type j: int
        :rtype: int
        """
        i, j = i + self.n, j + self.n
        sums = 0
        while i <= j:
            if i % 2 == 1: # 左边多出一个不能成对的
                sums += self.tree[i] 
                i += 1
            if j % 2 == 0: # 右边多出一个不能成对的
                sums += self.tree[j]
                j -= 1
            i //= 2
            j //= 2
        return sums