Leetcode Range Sum Query - Immutable

標籤：

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:

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

解題思路：

最簡單的dynamic Programming.  Complexity: NumArray O(n)   sumRange O(1)

為瞭解決邊界問題， 建立accu = new int[nums.length+1]   accu[i]表示nums[0]+...+ nums[i-1]

recurrence formula：accu[i] = accu[i-1] + nums[i-1]     base case: accu[0]= 0

Java code:

public class NumArray {    private int[] accu;    public NumArray(int[] nums) {        accu = new int[nums.length+1];        accu[0] = 0;        for(int i = 1; i<= nums.length; i++){            accu[i] = accu[i-1] + nums[i-1];        }    }    public int sumRange(int i, int j) {        return accu[j+1] - accu[i];    }}// Your NumArray object will be instantiated and called as such:// NumArray numArray = new NumArray(nums);// numArray.sumRange(0, 1);// numArray.sumRange(1, 2);

Reference:

1. https://leetcode.com/discuss/68725/5-lines-c-4-lines-python

2. https://leetcode.com/discuss/69081/3ms-java-solution-with-array-no-special-i-0-check

 

Leetcode Range Sum Query - Immutable