[LeetCode 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.

solution:

1.最初的想法比较直接，通过两个list来分别记录从root到node p 和q的path， 然后遍历list找到branch node的前一个就是公共parent。但是没有通过，以后有时间再实现

2。遍历判断两个node是否在左右子树中

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root == null) return null;
        if(root == p || root == q) return root;
        TreeNode l1 = lowestCommonAncestor(root.left, p, q);
        TreeNode l2 = lowestCommonAncestor(root.right, p, q);
        if(l1 !=null && l2!=null) return root;
        return l1 == null? l2 :l1;
    }