[LeetCode]303. Range Sum Query - Immutable

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

分析：根据给定的索引范围求和（动态规划）

解题思路:将0~n范围的数字求和，存入一维数组中。求索引i~j的和时只需要求得sum[j]-sum[i]

O(n) init O(1) query

public class Range_Sum_Query {

int sum[];
public Range_Sum_Query(int[] nums) {
if(nums!=null&&nums.length!=0){
sum = new int[nums.length];
sum[0] = nums[0];
for(int i=1;i<nums.length;i++){
sum[i] = sum[i-1]+nums[i];
}
}
}

public int sumRange(int i,int j){
if(i==0)return sum[j];
return sum[j]-sum[i-1];
}
}