[LeetCode][JavaScript]Lowest Common Ancestor of a Binary Tree
Lowest Common Ancestor of a Binary Tree
Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.
According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”
_______3______ / \ ___5__ ___1__ / \ / \ 6 _2 0 8 / \ 7 4
For example, the lowest common ancestor (LCA) of nodes 5
and 1
is 3
. Another example is LCA of nodes 5
and 4
is 5
, since a node can be a descendant of itself according to the LCA definition.
https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/
比较难以描述...
我的理解是把目标的节点向上"冒泡",拿这个做例子:the lowest common ancestor (LCA) of nodes 5
and 1
is 3
从下往上看,6这个节点往上一层返回null,因为这个节点本身,它的左子树和右子树都不包含1和5。
同理7,4,2返回null,
5这个节点左右子树不包含1和5,但它自身是5,返回5,
同理以1为根节点的树返回1,
以1为根节点的树发现左右都不为null,说明找到了。
以上是一种情况,还有一种情况就是一旦发现目前的节点等于p或者q,就可以直接返回,因为默认输入是正确的。
1 /** 2 * Definition for a binary tree node. 3 * function TreeNode(val) { 4 * this.val = val; 5 * this.left = this.right = null; 6 * } 7 */ 8 /** 9 * @param {TreeNode} root 10 * @param {TreeNode} p 11 * @param {TreeNode} q 12 * @return {TreeNode} 13 */ 14 var lowestCommonAncestor = function(root, p, q) { 15 if(root === null || root === p || root === q){ 16 return root; 17 } 18 var left = lowestCommonAncestor(root.left, p, q); 19 var right = lowestCommonAncestor(root.right, p, q); 20 return left !== null && right !== null ? root : (left !== null ? left : right); 21 }