CareerCup 4.7

4.7 Design an algorithm and write code to find the first common ancestor of two nodes in a binary tree. Avoid storing additional nodes in a data structure. NOTE: This is not necessarily a binary search tree.

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

TreeNode* LCA(TreeNode *root, TreeNode *a, TreeNode *b) {
    if (root == NULL) {
        return NULL;
    }
    if (root == a || root == b) {
        return root;
    }

    TreeNode *left = LCA(root->left, a, b);
    TreeNode *right = LCA(root->right, a, b);
    if (left && right) {
        return root;
    } else {
        return left ? left : right;
    }
}