Lowest Common Ancestor of a Binary Tree

LeetCode #236

Description:

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).”

Example:

        _______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.

Idea:

是Binary Tree而不是Binary Search Tree! 难度增加不少。

1. if root null, return null
2. if root==p or q, return root
3. search in left and right subtree
4. if both not null, return root
5. return the not null subtree

Code:

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if(root==NULL) return NULL;

        if(root==p || root==q) return root;

        TreeNode *left_LCA = lowestCommonAncestor(root->left, p, q);
        TreeNode *right_LCA = lowestCommonAncestor(root->right, p, q);

        if(left_LCA!=NULL && right_LCA!=NULL) return root;

        if(left_LCA!=NULL) return left_LCA;
        else return right_LCA;
    }
};