Method buildTree

java/0307-range-sum-query-mutable.java:12–20

(int[] nums)

    // O(n)
    private void buildTree(int[] nums) {
        // we build the tree from bottom up
        for (int i = n, j = 0; i < 2 * this.n; i++, j++) {
            this.tree[i] = nums[j];
        }
        for (int i = this.n-1; i > 0; i--) {
            this.tree[i] = this.tree[2 * i] + this.tree[2 * i + 1]; // parent value is sum of children values
        }
    }

    // O(logn)
    public void update(int index, int val) {