二叉查换树，左孩子小于等于根，右孩子大于根。
完全二叉树，除最后一层外，每一层上的节点数均达到最大值；在最后一层上只缺少右边的若干结点。 complete binary tree
满二叉树，完美二叉树是全部结点数达到最大的二叉树。perfect binary tree, full binary tree.
平衡二叉树，左右子树的高度在一定范围内。
二叉查找树（Binary Search Tree），也称二叉搜索树、有序二叉树（ordered binary tree），排序二叉树（sorted binary tree），是指一棵空树或者具有下列性质的二叉树：
- 若任意节点的左子树不空，则左子树上所有结点的值均小于或等于它的根结点的值；
- 任意节点的右子树不空，则右子树上所有结点的值均大于它的根结点的值；
- 任意节点的左、右子树也分别为二叉查找树。
- 没有键值相等的节点（no duplicate nodes）。
在二叉查找树删去一个结点，分三种情况讨论：
- 若*p结点为叶子结点，即PL(左子树)和PR(右子树)均为空树。由于删去叶子结点不破坏整棵树的结构，则只需修改其双亲结点的指针即可。
- 若*p结点只有左子树PL或右子树PR，此时只要令PL或PR直接成为其双亲结点*f的左子树（当*p是左子树）或右子树（当*p是右子树）即可，作此修改也不破坏二叉查找树的特性。
- 若*p结点的左子树和右子树均不空。在删去*p之后，为保持其它元素之间的相对位置不变，可按中序遍历保持有序进行调整，可以有两种做法：其一是令*p的左子树为*f的左/右(依*p是*f的左子树还是右子树而定)子树，*s为*p左子树的最右下的结点，而*p的右子树为*s的右子树；其二是令*p的直接前驱（in-order predecessor）或直接后继（in-order successor）替代*p，然后再从二叉查找树中删去它的直接前驱（或直接后继）。
4.1 Implement a function to check if a binary tree is balanced. For the purposes of this question, a balanced tree is defined to be a tree such that the heights of the two subtrees of any node never differ by more than one.
点此。
4.2 Given a directed graph, design an algorithm to find out whether there is a route between two nodes.
BFS或者DFS就行了，不过BFS可以避免在单条路径上挖得太深。
4.3 Given a sorted (increasing order) array with unique integer elements, write an algorithm to create a binary search tree with minimal height
点此。
4.4 Given a binary tree, design an algorithm which creates a linked list of all the nodes at each depth (e.g., if you have a tree with depth D, you'll have D linked lists).
BFS或者DFS都可以。注意的是，DFS虽然会用额外的O(lgn)的栈空间，但是整体的空间复杂度还是O(n)。
4.5 Implement a function to check if a binary tree is a binary search tree.
点此。BST要和中序遍历联系在一起。
4.6 Write an algorithm to find the 'next'node (i.e., in-order successor) of a given node in a binary search tree. You may assume that each node has a link to its parent.
如果有root结点，直接用中序遍历记录pre结点就可以做的。如果只有结点，那么需要分情况。
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.
leetcode上的博文，讲得很仔细。这道题很经典，尤其是有parent结点时的解法，类似求intersect linklist的交叉点。
4.8 You have two very large binary trees: Tl, with millions of nodes, and T2, with hundreds of nodes. Create an algorithm to decide if T2 is a subtree of Tl. A tree T2 is a subtree of Tl if there exists a node n in Tl such that the subtree of n is identical to T2. That is, if you cut off the tree at node n, the two trees would be identical.
这道题可以穷搜，也可以基于pre-order和in-order的travesal串，用子串查找。
4.9 You are given a binary tree in which each node contains a value. Design an algorithm to print all paths which sum to a given value. The path does not need to start or end at the root or a leaf.
只能穷搜了。
struct TreeNode {
TreeNode *left;
TreeNode *right;
TreeNode *parent;
int val;
TreeNode(int v):val(v),left(NULL),right(NULL),parent(NULL) {}
};
// 4.6
TreeNode* findInorderNext(TreeNode* node) {
if (node == NULL) {
return NULL;
} else if (node->right) {
node = node->right;
while (node->left) node = node->left;
return node;
} else {
while (node->parent && node->parent->left != node) node = node->parent;
return node->parent;
}
}
TreeNode* findPreordeNext(TreeNode* node) {
if (node == NULL) {
return NULL;
} else if (node->left) {
return node->left;
} else if (node->right) {
return node->right;
} else {
while (node->parent && node->parent->right == node) {
node = node->parent;
}
if (node->parent) return node->parent->right;
else return NULL;
}
}
TreeNode* findPostorderNext(TreeNode* node) {
if (node == NULL || node->parent == NULL) {
return NULL;
} else if (node->parent->right == node || node->parent->right == NULL) { // right subtree
return node->parent;
} else {
node = node->parent->right;
while (node->left) node = node->left;
return node;
}
}
// 4.7 (1) p, q in the BST, O(h) runtime
TreeNode* LCAInBST(TreeNode *root, TreeNode *p, TreeNode *q) {
if (!root || !p || !q) return NULL;
if (max(p->val, q->val) < root->val) {
return LCAInBST(root->left, p, q);
} else if (min(p->val, q->val) > root->val) {
return LCAInBST(root->right, p, q);
} else {
return root;
}
}
// 4.7 (2.1) p, q may not in the tree, this is just a binary tree
int matchCount(bool pExisted, bool qExisted) {
int m = ;
if (pExisted) m++;
if (qExisted) m++;
return m;
}
TreeNode* LCAInBT(TreeNode *root, TreeNode *p, TreeNode *q, bool &pExisted, bool &qExisted) {
if (!root) return NULL;
TreeNode* ret = LCAInBT(root->left, p, q, pExisted, qExisted);
int lm = matchCount(pExisted, qExisted);
if (lm == ) return ret; // p, q are in the left subtree
ret = LCAInBT(root->right, p, q, pExisted, qExisted);
int rm = matchCount(pExisted, qExisted);
if (rm == ) { // p, q are found
if (lm == ) return root; //p, q are in different subtree, thus return root
else return ret; // lm == 0, p, q are in the right subtree
}
if (root == p) pExisted = true;
if (root == q) qExisted = true;
if (pExisted && qExisted) return root; // if q is a child of q (or, q is a child of p)
return NULL; // if p or q is not existed
}
// 4.7(2.2) the parent node is available
int getHeight(TreeNode* p) {
int h = ;
while (p) {
p = p->parent;
h++;
}
return h;
}
TreeNode* LCAInBT(TreeNode *p, TreeNode *q) {
if (!p || !q) return NULL;
int h1 = getHeight(p);
int h2 = getHeight(q);
if (h1 > h2) {
swap(p, q);
swap(h1, h2);
}
for (int i = ; i < h2 - h1; ++i) {
q = q->parent;
}
while (p && q) {
if (p == q) return p;
p = p->parent;
q = q->parent;
}
return NULL;
}