[Java]Leetcode236 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.

 题意：之前一题的树是二叉查找树，而本题的树就是一棵普通的二叉树。给定两个节点，判断这两个节点的相同的父节点。父节点也有可能是两个节点其中之一。这一题，感觉用递归方法解题更简单。

解题思路：先遍历左子树，返回匹配的点，没有返回null。后遍历右子树，返回匹配的点，没有返回null。如果left和right都不为null，则根节点是它们的共同节点。如果left为空，则两个节点都在右子树，返回right。如果right为空，则两个节点都在左子树，返回left.

 public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root==null)return null;
        if(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;
    }