236. 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.

采用迭代的方法，当root == null, p, q的时候，返回root，否则就遍历左右节点，如果左右都找到了，说明这个节点是pq的公共节点，返回root，如果left有，right 没有，返回left，如果right有，left没有，返回right。代码如下：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public 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? right: left;    }}