[LeetCode]--303. Range Sum Query - Immutable

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.

我最开始用的就是把数组的元素相加，因为知道i，j嘛，可以依次相加，但是这样就会超时。

如果最开始在构造函数里面构造数组的时候就把和都存进去，这样既可。

public class NumArray {

    public int[] nums;

    public NumArray(int[] nums) {
        if (nums.length == 0)
            return;
        this.nums = new int[nums.length];
        Arrays.fill(this.nums, 0);
        this.nums[0] = nums[0];
        for (int i = 1; i < nums.length; i++) {
            this.nums[i] = this.nums[i - 1] + nums[i];
        }
    }

    public int sumRange(int i, int j) {
        if (this.nums.length == 0)
            return 0;
        if (j > this.nums.length - 1)
            j = this.nums.length - 1;
        if (i == 0)
            return this.nums[j];
        return this.nums[j] - this.nums[i - 1];
    }
}