LeetCode题目

654.最大二叉树

题目链接：Maximum Binary Tree - LeetCode

代码如下：

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func constructMaximumBinaryTree(nums []int) *TreeNode {
	if len(nums) == 0 {
		return nil
	}
	root := &TreeNode{
		Val:   0,
		Left:  nil,
		Right: nil,
	}
	if len(nums) == 1 {
		root.Val = nums[0]
		return root
	}
	maxIndex := -1
	maxValue := -1
	for index, value := range nums {
		if value > maxValue {
			maxIndex = index
			maxValue = value
		}
	}
	root.Val = maxValue
	leftNums := nums[:maxIndex]
	rightNums := nums[maxIndex+1:]
	root.Left = constructMaximumBinaryTree(leftNums)
	root.Right = constructMaximumBinaryTree(rightNums)
	return root
}

617.合并二叉树

题目链接：Merge Two Binary Trees - LeetCode

代码如下：

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func mergeTrees(root1 *TreeNode, root2 *TreeNode) *TreeNode {
	if root1 == nil {
		return root2
	}
	if root2 == nil {
		return root1
	}
	root1.Val += root2.Val
	root1.Left = mergeTrees(root1.Left, root2.Left)
	root1.Right = mergeTrees(root1.Right, root2.Right)
	return root1
}

700.二叉搜索树中的搜索

题目链接：Search in a Binary Search Tree - LeetCode

代码如下：

递归法

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func searchBST(root *TreeNode, val int) *TreeNode {
	if root == nil || root.Val == val {
		return root
	}
	var result *TreeNode
	if root.Val > val {
		result = searchBST(root.Left, val)
	} else {
		result = searchBST(root.Right, val)
	}
	return result
}

迭代法

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func searchBST(root *TreeNode, val int) *TreeNode {
	for root != nil {
		if root.Val > val {
			root = root.Left
		} else if root.Val < val {
			root = root.Right
		} else {
			return root
		}
	}
	return nil
}

98.验证二叉搜索树

题目链接：Validate Binary Search Tree - LeetCode

代码如下：

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func isValidBST(root *TreeNode) bool {
	var pre *TreeNode
	var travel func(node *TreeNode) bool
	travel = func(node *TreeNode) bool {
		if node == nil {
			return true
		}
		left := travel(node.Left)
		if pre != nil && pre.Val >= node.Val {
			return false
		}
		pre = node
		right := travel(node.Right)
		return left && right
	}
	return travel(root)
}

总结

• 二叉搜索树满足以下条件

1. 对于根节点，左子树中所有节点的值 < 根节点的值 < 右子树中所有节点的值
2. 任意节点的左、右子树也是二叉搜索树，即同样满足条件1.

• 二叉搜索树的中序遍历序列是升序的，因此遇到二叉搜索树问题，多考虑使用中序遍历