【Medium】203. Segment Tree Modify

For a Maximum Segment Tree, which each node has an extra value max to store the maximum value in this node's interval.

Implement a modify function with three parameter root, index and value to change the node's value with [start, end] = [index, index] to the new given value. Make sure after this change, every node in segment tree still has the max attribute with the correct value.

Notice:

We suggest you finish problem Segment Tree Build and Segment Tree Query first.

Example:

For segment tree:

                         [1, 4, max=3]
                      /                \
          [1, 2, max=2]                [3, 4, max=3]
         /              \             /             \
  [1, 1, max=2], [2, 2, max=1], [3, 3, max=0], [4, 4, max=3]

if call modify(root, 2, 4), we can get:

                         [1, 4, max=4]
                      /                \
          [1, 2, max=4]                [3, 4, max=3]
         /              \             /             \
  [1, 1, max=2], [2, 2, max=4], [3, 3, max=0], [4, 4, max=3]

or call modify(root, 4, 0), we can get:

                         [1, 4, max=2]
                      /                \
          [1, 2, max=2]                [3, 4, max=0]
         /              \             /             \
  [1, 1, max=2], [2, 2, max=1], [3, 3, max=0], [4, 4, max=0]

解题思路

略。

核心代码

    if (root == null || index < root.start || index > root.end) return;
    if (root.start == index && root.end == index) {
        root.max = value;
        return;
    }

    modify(root.left, index, value);
    modify(root.right, index, value);

    root.max = Math.max(root.left.max, root.right.max);

时间空间复杂度

O(logn)

n为end-start