先上二叉树的数据结构：
class TreeNode{
int val;
//左孩子
TreeNode left;
//右孩子
TreeNode right;
}
二叉树的题目普遍可以用递归和迭代的方式来解。
1.求二叉树的最大深度
int maxDeath(TreeNode node){
if(node==null){
return 0;
}
int left = maxDeath(node.left);
int right = maxDeath(node.right);
return Math.max(left,right) + 1;
}
2.求二叉树的最小深度
int getMinDepth(TreeNode root){
if(root == null){
return 0;
}
return getMin(root);
}
int getMin(TreeNode root){
if(root == null){
return Integer.MAX_VALUE;
}
if(root.left == null&&root.right == null){
return 1;
}
return Math.min(getMin(root.left),getMin(root.right)) + 1;
}
3,求二叉树中节点的个数
int numOfTreeNode(TreeNode root){
if(root == null){
return 0;
}
int left = numOfTreeNode(root.left);
int right = numOfTreeNode(root.right);
return left + right + 1;
}
4,求二叉树中叶子节点的个数
int numsOfNoChildNode(TreeNode root){
if(root == null){
return 0;
}
if(root.left==null&&root.right==null){
return 1;
}
return numsOfNodeTreeNode(root.left)+numsOfNodeTreeNode(root.right);
}
5.求二叉树中第k层节点的个数
int numsOfkLevelTreeNode(TreeNode root,int k){
if(root == null||k<1){
return 0;
}
if(k==1){
return 1;
}
int numsLeft = numsOfkLevelTreeNode(root.left,k-1);
int numsRight = numsOfkLevelTreeNode(root.right,k-1);
return numsLeft + numsRight;
}
6.判断二叉树是否是平衡二叉树
boolean isBalanced(TreeNode node){
return maxDeath2(node)!=-1;
}
int maxDeath2(TreeNode node){
if(node == null){
return 0;
}
int left = maxDeath2(node.left);
int right = maxDeath2(node.right);
if(left==-1||right==-1||Math.abs(left-right)>1){
return -1;
}
return Math.max(left, right) + 1;
}
7.判断二叉树是否是完全二叉树
什么是完全二叉树呢？参见
boolean isCompleteTreeNode(TreeNode root){
if(root == null){
return false;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.add(root);
boolean result = true;
boolean hasNoChild = false;
while(!queue.isEmpty()){
TreeNode current = queue.remove();
if(hasNoChild){
if(current.left!=null||current.right!=null){
result = false;
break;
}
}else{
if(current.left!=null&¤t.right!=null){
queue.add(current.left);
queue.add(current.right);
}else if(current.left!=null&¤t.right==null){
queue.add(current.left);
hasNoChild = true;
}else if(current.left==null&¤t.right!=null){
result = false;
break;
}else{
hasNoChild = true;
}
}
}
return result;
}
8.两个二叉树是否完全相同
boolean isSameTreeNode(TreeNode t1,TreeNode t2){
if(t1==null&&t2==null){
return true;
}
else if(t1==null||t2==null){
return false;
}
if(t1.val != t2.val){
return false;
}
boolean left = isSameTreeNode(t1.left,t2.left);
boolean right = isSameTreeNode(t1.right,t2.right);
return left&&right;
}
9.两个二叉树是否互为镜像
boolean isMirror(TreeNode t1,TreeNode t2){
if(t1==null&&t2==null){
return true;
}
if(t1==null||t2==null){
return false;
}
if(t1.val != t2.val){
return false;
}
return isMirror(t1.left,t2.right)&&isMirror(t1.right,t2.left);
}
10.翻转二叉树or镜像二叉树
TreeNode mirrorTreeNode(TreeNode root){
if(root == null){
return null;
}
TreeNode left = mirrorTreeNode(root.left);
TreeNode right = mirrorTreeNode(root.right);
root.left = right;
root.right = left;
return root;
}
11.求两个二叉树的最低公共祖先节点
TreeNode getLastCommonParent(TreeNode root,TreeNode t1,TreeNode t2){
if(findNode(root.left,t1)){
if(findNode(root.right,t2)){
return root;
}else{
return getLastCommonParent(root.left,t1,t2);
}
}else{
if(findNode(root.left,t2)){
return root;
}else{
return getLastCommonParent(root.right,t1,t2)
}
}
}
// 查找节点node是否在当前 二叉树中
boolean findNode(TreeNode root,TreeNode node){
if(root == null || node == null){
return false;
}
if(root == node){
return true;
}
boolean found = findNode(root.left,node);
if(!found){
found = findNode(root.right,node);
}
return found;
}
12.二叉树的前序遍历
迭代解法
ArrayList<Integer> preOrder(TreeNode root){
Stack<TreeNode> stack = new Stack<TreeNode>();
ArrayList<Integer> list = new ArrayList<Integer>();
if(root == null){
return list;
}
stack.push(root);
while(!stack.empty()){
TreeNode node = stack.pop();
list.add(node.val);
if(node.right!=null){
stack.push(node.right);
}
if(node.left != null){
stack.push(node.left);
}
}
return list;
}
递归解法
ArrayList<Integer> preOrderReverse(TreeNode root){
ArrayList<Integer> result = new ArrayList<Integer>();
preOrder2(root,result);
return result;
}
void preOrder2(TreeNode root,ArrayList<Integer> result){
if(root == null){
return;
}
result.add(root.val);
preOrder2(root.left,result);
preOrder2(root.right,result);
}
13.二叉树的中序遍历
ArrayList<Integer> inOrder(TreeNode root){
ArrayList<Integer> list = new ArrayList<<Integer>();
Stack<TreeNode> stack = new Stack<TreeNode>();
TreeNode current = root;
while(current != null|| !stack.empty()){
while(current != null){
stack.add(current);
current = current.left;
}
current = stack.peek();
stack.pop();
list.add(current.val);
current = current.right;
}
return list;
}
14.二叉树的后序遍历
ArrayList<Integer> postOrder(TreeNode root){
ArrayList<Integer> list = new ArrayList<Integer>();
if(root == null){
return list;
}
list.addAll(postOrder(root.left));
list.addAll(postOrder(root.right));
list.add(root.val);
return list;
}
15.前序遍历和后序遍历构造二叉树
TreeNode buildTreeNode(int[] preorder,int[] inorder){
if(preorder.length!=inorder.length){
return null;
}
return myBuildTree(inorder,0,inorder.length-1,preorder,0,preorder.length-1);
}
TreeNode myBuildTree(int[] inorder,int instart,int inend,int[] preorder,int prestart,int preend){
if(instart>inend){
return null;
}
TreeNode root = new TreeNode(preorder[prestart]);
int position = findPosition(inorder,instart,inend,preorder[start]);
root.left = myBuildTree(inorder,instart,position-1,preorder,prestart+1,prestart+position-instart);
root.right = myBuildTree(inorder,position+1,inend,preorder,position-inend+preend+1,preend);
return root;
}
int findPosition(int[] arr,int start,int end,int key){
int i;
for(i = start;i<=end;i++){
if(arr[i] == key){
return i;
}
}
return -1;
}
16.在二叉树中插入节点
TreeNode insertNode(TreeNode root,TreeNode node){
if(root == node){
return node;
}
TreeNode tmp = new TreeNode();
tmp = root;
TreeNode last = null;
while(tmp!=null){
last = tmp;
if(tmp.val>node.val){
tmp = tmp.left;
}else{
tmp = tmp.right;
}
}
if(last!=null){
if(last.val>node.val){
last.left = node;
}else{
last.right = node;
}
}
return root;
}
17.输入一个二叉树和一个整数，打印出二叉树中节点值的和等于输入整数所有的路径
void findPath(TreeNode r,int i){
if(root == null){
return;
}
Stack<Integer> stack = new Stack<Integer>();
int currentSum = 0;
findPath(r, i, stack, currentSum);
}
void findPath(TreeNode r,int i,Stack<Integer> stack,int currentSum){
currentSum+=r.val;
stack.push(r.val);
if(r.left==null&&r.right==null){
if(currentSum==i){
for(int path:stack){
System.out.println(path);
}
}
}
if(r.left!=null){
findPath(r.left, i, stack, currentSum);
}
if(r.right!=null){
findPath(r.right, i, stack, currentSum);
}
stack.pop();
}
18.二叉树的搜索区间
给定两个值 k1 和 k2（k1 < k2）和一个二叉查找树的根节点。找到树中所有值在 k1 到 k2 范围内的节点。即打印所有x (k1 <= x <= k2) 其中 x 是二叉查找树的中的节点值。返回所有升序的节点值。
ArrayList<Integer> result;
ArrayList<Integer> searchRange(TreeNode root,int k1,int k2){
result = new ArrayList<Integer>();
searchHelper(root,k1,k2);
return result;
}
void searchHelper(TreeNode root,int k1,int k2){
if(root == null){
return;
}
if(root.val>k1){
searchHelper(root.left,k1,k2);
}
if(root.val>=k1&&root.val<=k2){
result.add(root.val);
}
if(root.val<k2){
searchHelper(root.right,k1,k2);
}
}
19.二叉树的层次遍历
ArrayList<ArrayList<Integer>> levelOrder(TreeNode root){
ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
if(root == null){
return result;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while(!queue.isEmpty()){
int size = queue.size();
ArrayList<<Integer> level = new ArrayList<Integer>():
for(int i = 0;i < size ;i++){
TreeNode node = queue.poll();
level.add(node.val);
if(node.left != null){
queue.offer(node.left);
}
if(node.right != null){
queue.offer(node.right);
}
}
result.add(Level);
}
return result;
}
20.二叉树内两个节点的最长距离
二叉树中两个节点的最长距离可能有三种情况：
1.左子树的最大深度+右子树的最大深度为二叉树的最长距离
2.左子树中的最长距离即为二叉树的最长距离
3.右子树种的最长距离即为二叉树的最长距离
因此，递归求解即可
private static class Result{
int maxDistance;
int maxDepth;
public Result() {
}
public Result(int maxDistance, int maxDepth) {
this.maxDistance = maxDistance;
this.maxDepth = maxDepth;
}
}
int getMaxDistance(TreeNode root){
return getMaxDistanceResult(root).maxDistance;
}
Result getMaxDistanceResult(TreeNode root){
if(root == null){
Result empty = new Result(0,-1);
return empty;
}
Result lmd = getMaxDistanceResult(root.left);
Result rmd = getMaxDistanceResult(root.right);
Result result = new Result();
result.maxDepth = Math.max(lmd.maxDepth,rmd.maxDepth) + 1;
result.maxDistance = Math.max(lmd.maxDepth + rmd.maxDepth,Math.max(lmd.maxDistance,rmd.maxDistance));
return result;
}
21.不同的二叉树
给出 n，问由 1...n 为节点组成的不同的二叉查找树有多少种？
int numTrees(int n ){
int[] counts = new int[n+2];
counts[0] = 1;
counts[1] = 1;
for(int i = 2;i<=n;i++){
for(int j = 0;j<i;j++){
counts[i] += counts[j] * counts[i-j-1];
}
}
return counts[n];
}
22.判断二叉树是否是合法的二叉查找树(BST)
一棵BST定义为：
节点的左子树中的值要严格小于该节点的值。
节点的右子树中的值要严格大于该节点的值。
左右子树也必须是二叉查找树。
一个节点的树也是二叉查找树。
public int lastVal = Integer.MAX_VALUE;
public boolean firstNode = true;
public boolean isValidBST(TreeNode root) {
// write your code here
if(root==null){
return true;
}
if(!isValidBST(root.left)){
return false;
}
if(!firstNode&&lastVal >= root.val){
return false;
}
firstNode = false;
lastVal = root.val;
if (!isValidBST(root.right)) {
return false;
}
return true;
}
深刻的理解这些题的解法思路，在面试中的二叉树题目就应该没有什么问题。
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