LeetCode-in-Java

307. Range Sum Query - Mutable

Medium

Given an integer array nums, handle multiple queries of the following types:

1. Update the value of an element in nums.
2. Calculate the sum of the elements of nums between indices left and right inclusive where left <= right.

Implement the NumArray class:

• NumArray(int[] nums) Initializes the object with the integer array nums.
• void update(int index, int val) Updates the value of nums[index] to be val.
• int sumRange(int left, int right) Returns the sum of the elements of nums between indices left and right inclusive (i.e. nums[left] + nums[left + 1] + ... + nums[right]).

Example 1:

Input

["NumArray", "sumRange", "update", "sumRange"]
[[[1, 3, 5]], [0, 2], [1, 2], [0, 2]]

Output: [null, 9, null, 8]

Explanation:

NumArray numArray = new NumArray([1, 3, 5]);
numArray.sumRange(0, 2); // return 1 + 3 + 5 = 9
numArray.update(1, 2); // nums = [1, 2, 5]
numArray.sumRange(0, 2); // return 1 + 2 + 5 = 8 

Constraints:

• 1 <= nums.length <= 3 * 104
• -100 <= nums[i] <= 100
• 0 <= index < nums.length
• -100 <= val <= 100
• 0 <= left <= right < nums.length
• At most 3 * 104 calls will be made to update and sumRange.

Solution

public class NumArray {
    private int[] tree;
    private int[] nums;

    public NumArray(int[] nums) {
        tree = new int[nums.length + 1];
        this.nums = nums;
        // copy the array into the tree
        System.arraycopy(nums, 0, tree, 1, nums.length);
        for (int i = 1; i < tree.length; i++) {
            int parent = i + (i & -i);
            if (parent < tree.length) {
                tree[parent] += tree[i];
            }
        }
    }

    public void update(int index, int val) {
        int currValue = nums[index];
        nums[index] = val;
        index++;
        while (index < tree.length) {
            tree[index] = tree[index] - currValue + val;
            index = index + (index & -index);
        }
    }

    private int sum(int i) {
        int sum = 0;
        while (i > 0) {
            sum += tree[i];
            i -= (i & -i);
        }
        return sum;
    }

    public int sumRange(int left, int right) {
        return sum(right + 1) - sum(left);
    }
}

/*
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * obj.update(index,val);
 * int param_2 = obj.sumRange(left,right);
 */

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