二叉搜索树

TIP

这里需要注意，整颗树左侧都必须小于根节点，右侧也如此；而不仅仅是左节点小于根节点，右节点大于根节点

错误解法：

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isValidBST = function (root) {
  if (root === null) return false
  function traversal(node) {
    if (node === null) return null
    const left = traversal(node.left)
    const right = traversal(node.right)
    if (left === false || right === false) {
      return false
    } else if (left !== null && left >= node.val) {
      return false
    } else if (right !== null && right <= node.val) {
      return false
    }
    return node.val
  }
  return traversal(root) !== false
};

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isValidBST = function (root) {
  const compare = (root, lower, upper) => {
    if (root === null) {
      return true
    }
    if (root.val <= lower || root.val >= upper) {
      return false
    }
    return compare(root.left, lower, root.val) && compare(root.right, root.val, upper)
  }
  return compare(root, -Infinity, Infinity)
};

二叉搜索树#

二叉搜索树