#203 Segment Tree Modify

最新推荐文章于 2022-01-14 21:43:29 发布

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本文介绍了一种在最大值段树中修改指定节点值的方法，并确保更新后树的每个节点仍保持正确的最大值属性。通过递归算法实现节点值的更新。

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题目描述：

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.

Have you met this question in a real interview?

Yes

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]

Challenge

Do it in O(h) time, h is the height of the segment tree.

题目思路：

题目要求更改max value，只有子树的max value都改好了，才能知道当前root的max value是个啥。用recursion就可以了。

Mycode（AC = 104ms）：

/**
 * Definition of SegmentTreeNode:
 * class SegmentTreeNode {
 * public:
 *     int start, end, max;
 *     SegmentTreeNode *left, *right;
 *     SegmentTreeNode(int start, int end, int max) {
 *         this->start = start;
 *         this->end = end;
 *         this->max = max;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     *@param root, index, value: The root of segment tree and 
     *@ change the node's value with [index, index] to the new given value
     *@return: void
     */
    void modify(SegmentTreeNode *root, int index, int value) {
        // write your code here
        if (index < root->start || index > root->end) {
            return;
        }
        else if (root->start == index && root->end == index) {
            root->max = value;
        }
        else if (root->start == root->end) {
            return;
        }
        else {
            int modified_max = INT_MIN;
            if (root->left) {
                modify(root->left, index, value);
                modified_max = max(modified_max, root->left->max);
            }
            if (root->right) {
                modify(root->right, index, value);
                modified_max = max(modified_max, root->right->max);
            }
            
            root->max = modified_max;
            
        }
        
    }
};