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.

题目含义：求一个普通二叉树中两个节点的最小公共父节点

 

    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || p == root || q == root) return root;
        TreeNode left = lowestCommonAncestor(root.left, p, q);
        TreeNode right = lowestCommonAncestor(root.right, p , q);
        if (left!=null && right!=null) return root;  //p和q与root的左右子节点相等，那么root即为p和q的公共父节点
        return left!=null ? left : right;//p和q至少有一个为空，表示p和q落在了root的同一侧子树中，所以应该返回不为空的那个子树节点     
    }

 类似题目：235. Lowest Common Ancestor of a Binary Search Tree