LeetCode OJ 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.

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解答

这题有可能会时间超限，就是因为第一遍做的时候没有将两个递归调用的结果用局部变量保存，导致了多次调用的情况出现……

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode* p, struct TreeNode* q) {
    struct TreeNode *left, *right;
    if(NULL == root){
            return NULL;
        }
        if(root == p||root == q){
            return root;
        }
        left = lowestCommonAncestor(root->left, p, q);
        right = lowestCommonAncestor(root->right, p, q);
        if(NULL == left){
            return right;
        }
        if(NULL == right){
            return left;
        }
        return root;
}