lc303. Range Sum Query - Immutable

303. Range Sum Query - Immutable Easy

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Share 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.

思路：用數組dp記錄前n位的和，sumRange(i,j)即爲計算dp[j]-dp[i-1]的值，注意i爲0的狀況

代碼：python3

class NumArray:

    def __init__(self, nums: List[int]):
        self.dp=nums
        for i in range(len(nums)):
            if i==0:
                self.dp[i]=nums[i]
            else:
                self.dp[i]=self.dp[i-1]+nums[i]
    def sumRange(self, i: int, j: int) -> int:
        if i==0:
            return self.dp[j]
        return self.dp[j]-self.dp[i-1]
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