題目
Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.
Example 1:
2
/ \
1 3
Binary tree [2,1,3], return true.
答案
class Solution {
public boolean isValidBST(TreeNode root) {
List<Integer> list = new ArrayList<>();
dumpList(root, list);
for(int i = 0; i < list.size() - 1; i++) {
if(list.get(i) >= list.get(i + 1)) return false;
}
return true;
}
public void dumpList(TreeNode root, List<Integer> list) {
if(root == null) return;
dumpList(root.left, list);
list.add(root.val);
dumpList(root.right, list);
}
}
