二叉树最近公共父节点

二叉树最近公共父节点大致分为两种，一种是二叉查找树，另一种是普通二叉树，相对来说二叉查找树更简单，因为我们能利用二叉查找树的特性来更快的找到最近公共父节点

自己构造的树节点类

data class TreeNode(
    val left: TreeNode? = null,
    val right: TreeNode? = null,
    val value: Int = 0
)

二叉查找树的最近公共父节点

目标二叉查找树：

val node =
    TreeNode(
        TreeNode(
            TreeNode(TreeNode(value = 3), TreeNode(value = 5), 4),
            TreeNode(TreeNode(value = 8), TreeNode(value = 10), 9),
        7),
        TreeNode(
            TreeNode(TreeNode(value = 13), TreeNode(value = 16), 15),
            TreeNode(TreeNode(value = 17), TreeNode(value = 21), 20),
        18),
    12)

查找算法

主要利用了每个节点的左节点都小于自己，每个右节点都大于自己的特性，递归查找目标节点

fun getNearestCommonAncestorInBinarySearchTree(node: TreeNode?, one: TreeNode, another: TreeNode): TreeNode? {
    if (node == null || node.value == one.value || node.value == another.value) return node
    if (node.value > one.value  && node.value > another.value) return getNearestCommonAncestorInBinarySearchTree(node.left, one, another)
    if (node.value < one.value  && node.value < another.value) return getNearestCommonAncestorInBinarySearchTree(node.right, one, another)
    return node
}

二叉树的最近公共父节点

目标二叉查找树：

val node =
    TreeNode(
        TreeNode(
            TreeNode(TreeNode(value = 55), TreeNode(value = 12), 35),
            TreeNode(TreeNode(value = 85), TreeNode(value = 79), 46),
        72),
        TreeNode(
            TreeNode(TreeNode(value = 1), TreeNode(value = 23), 42),
            TreeNode(TreeNode(value = 4), TreeNode(value = 99), 502),
        64),
    89)

查找算法

二叉树没有上面的查找树的特性，需要其他判断。只要最终判断一个节点在某个节点的左侧，另一个在其右侧，则该节点为目标节点

fun getNearestCommonAncestorInBinaryTree(node: TreeNode?, one: TreeNode, another: TreeNode): TreeNode? {
    if (node == null || node.value == one.value || node.value == another.value) return node
    val left = getNearestCommonAncestorInBinaryTree(node.left, one, another)
    val right = getNearestCommonAncestorInBinaryTree(node.right, one, another)
    if (left != null && right != null) return node
    return left ?: right
}