C++ in 13 lines

  • 10

    There are just two cases:

    1. The easier one: p has right subtree, then its successor is just the leftmost child of its right subtree;
    2. The harder one: p has no right subtree, then a traversal is needed to find its successor.

    Traversal: we start from the root, each time we see a node with val larger than p -> val, we know this node may be p's successor. So we record it in suc. Then we try to move to the next level of the tree: if p -> val > root -> val, which means p is in the right subtree, then its successor is also in the right subtree, so we update root = root -> right; if p -> val < root -> val, we update root = root -> left similarly; once we find p -> val == root -> val, we know we've reached at p and the current suc is just its successor.

    The code is as follows. You may try some examples to see how it works :-)

    class Solution {
        TreeNode* inorderSuccessor(TreeNode* root, TreeNode* p) {
            if (p -> right) return leftMost(p -> right);
            TreeNode* suc = NULL;
            while (root) {
                if (p -> val < root -> val) {
                    suc = root;
                    root = root -> left;
                else if (p -> val > root -> val)
                    root = root -> right; 
                else break;
            return suc;
        TreeNode* leftMost(TreeNode* node) {
            while (node -> left) node = node -> left;
            return node;

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