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IsValidBST98.java
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IsValidBST98.java
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package Leetcode;
import java.util.Deque;
import java.util.LinkedList;
import java.util.Stack;
public class IsValidBST98 {
/**Method 1: Recursive; Time:O(N); Space: O(N) **/
public boolean isValidBST(TreeNode root) {
return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE); //注意是long型
}
private boolean isValidBST(TreeNode root, long lower, long high) {
if(root == null) {
return true;
}
if(root.val <= lower || root.val >= high) {
return false;
}
return isValidBST(root.left, lower, root.val) && isValidBST(root.right, root.val, high);
}
/**Method 2: Iteration; Time:O(N); Space: O(N) **/
public boolean isValidBST2(TreeNode root) {
Stack<TreeNode> s = new Stack<>();
long lastV = -Long.MAX_VALUE;
while(!s.isEmpty() || root!=null) { //注意是long型
while(root!=null) {
s.push(root);
root = root.left;
}
root = s.pop();
if(root.val <= lastV){
return false;
}
lastV = root.val;
root = root.right;
}
return true;
}
//Definition for a binary tree node.
public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode() {}
TreeNode(int val) { this.val = val; }
TreeNode(int val, TreeNode left, TreeNode right) {
this.val = val;
this.left = left;
this.right = right;
}
}
}