LeetCode：M-236. Lowest Common Ancestor of a Binary Tree

LeetCode链接

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.

1、判断当前node是否等于p,q，若相等，则当前node为目前p or q 的最近father（其实不应该这样，应该是前一个，但是题目中实验发现，同一边的p和q，LCA为第一个发现的p or q）

2、若当前节点的左右都找到了p/q中的一个node，则当前节点是LCA

3、否则LCA为任一个不为null的left or right（即使两者均为null，最后的返回式也成立）

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {          if(root==null || root==p || root==q)             return root;                TreeNode left = lowestCommonAncestor(root.left, p, q);        TreeNode right = lowestCommonAncestor(root.right, p, q);                if(left!=null && right!=null)            return root;                return left!=null?left:right;    }}