递归解法
/** * Definition for binary tree * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public boolean isValidBST(TreeNode root) { return isValidBSTHelper(root, Integer.MIN_VALUE, Integer.MAX_VALUE); } public boolean isValidBSTHelper(TreeNode root, int low, int high){ if(root == null){ return true; } int value = root.val; if( low < value && value < high && isValidBSTHelper(root.left, low, value) && isValidBSTHelper(root.right, value, high)){ return true; }else{ return false; } } }
In order traversal 遍历树 将val添加到数组里，然后遍历一遍数组看看数组是否是递增的
/** * Definition for binary tree * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public boolean isValidBST(TreeNode root) { if(root == null){ return true; } ArrayList<Integer> result = new ArrayList<Integer>(); inOrder(root, result); for(int i = 1; i< result.size(); i++){ /*注意 root的值不能和左节点或有节点相等*/ if(result.get(i-1) >= result.get(i)){ return false; } } return true; } public void inOrder(TreeNode root, ArrayList<Integer> result){ if(root == null){ return; } if(root.left != null){ inOrder(root.left, result); } result.add(root.val); if(root.right != null){ inOrder(root.right, result); } } }