1.二叉树基本概念见上节：《Java 二叉树基础概念（递归&迭代）》
2.本次展示链式存储
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以此图为例，完整代码如下：
//基础二叉树实现 //使用左右孩子表示法 import java.util.*; import java.util.Deque; public class myBinTree { private static class TreeNode{ char val; TreeNode left; TreeNode right; public TreeNode(char val) { this.val = val; } } public static TreeNode build(){ TreeNode nodeA=new TreeNode('A'); TreeNode nodeB=new TreeNode('B'); TreeNode nodeC=new TreeNode('C'); TreeNode nodeD=new TreeNode('D'); TreeNode nodeE=new TreeNode('E'); TreeNode nodeF=new TreeNode('F'); TreeNode nodeG=new TreeNode('G'); TreeNode nodeH=new TreeNode('H'); nodeA.left=nodeB; nodeA.right=nodeC; nodeB.left=nodeD; nodeB.right=nodeE; nodeE.right=nodeH; nodeC.left=nodeF; nodeC.right=nodeG; return nodeA; } //方法1(递归) //先序遍历: 根左右 public static void preOrder(TreeNode root){ if(root==null){ return; } System.out.print(root.val+" "); preOrder(root.left); preOrder(root.right); } //方法1(递归) //中序遍历 public static void inOrder(TreeNode root){ if(root==null){ return; } inOrder(root.left); System.out.print(root.val+" "); inOrder(root.right); } //方法1(递归) //后序遍历 public static void postOrder(TreeNode root){ if(root==null){ return; } postOrder(root.left); postOrder(root.right); System.out.print(root.val+" "); } //方法2(迭代) //先序遍历 (迭代) public static void preOrderNonRecursion(TreeNode root){ if(root==null){ return ; } Deque<TreeNode> stack=new LinkedList<>(); stack.push(root); while (!stack.isEmpty()){ TreeNode cur=stack.pop(); System.out.print(cur.val+" "); if(cur.right!=null){ stack.push(cur.right); } if(cur.left!=null){ stack.push(cur.left); } } } //方法2(迭代) //中序遍历 (迭代) public static void inorderTraversalNonRecursion(TreeNode root) { if(root==null){ return ; } Deque<TreeNode> stack=new LinkedList<>(); // 当前走到的节点 TreeNode cur=root; while (!stack.isEmpty() || cur!=null){ // 不管三七二十一，先一路向左走到根儿~ while (cur!=null){ stack.push(cur); cur=cur.left; } // 此时cur为空，说明走到了null，此时栈顶就存放了左树为空的节点 cur=stack.pop(); System.out.print(cur.val+" "); // 继续访问右子树 cur=cur.right; } } //方法2(迭代) //后序遍历 (迭代) public static void postOrderNonRecursion(TreeNode root){ if(root==null){ return; } Deque<TreeNode> stack=new LinkedList<>(); TreeNode cur=root; TreeNode prev=null; while (!stack.isEmpty() || cur!=null){ while (cur!=null){ stack.push(cur); cur=cur.left; } cur=stack.pop(); if(cur.right==null || prev==cur.right){ System.out.print(cur.val+" "); prev=cur; cur=null; }else { stack.push(cur); cur=cur.right; } } } //方法1(递归) //传入一颗二叉树的根节点，就能统计出当前二叉树中一共有多少个节点，返回节点数 //此时的访问就不再是输出节点值，而是计数器 + 1操作 public static int getNodes(TreeNode root){ if(root==null){ return 0; } return 1+getNodes(root.left)+getNodes(root.right); } //方法2(迭代) //使用层序遍历来统计当前树中的节点个数 public static int getNodesNoRecursion(TreeNode root){ if(root==null){ return 0; } int size=0; Deque<TreeNode> queue=new LinkedList<>(); queue.offer(root); while (!queue.isEmpty()) { TreeNode cur = queue.poll(); size++; if (cur.left != null) { queue.offer(cur.left); } if (cur.right != null) { queue.offer(cur.right); } } return size; } //方法1(递归) //传入一颗二叉树的根节点，就能统计出当前二叉树的叶子结点个数 public static int getLeafNodes(TreeNode root){ if(root==null){ return 0; } if(root.left==null && root.right==null){ return 1; } return getLeafNodes(root.left)+getLeafNodes(root.right); } //方法2(迭代) //使用层序遍历来统计叶子结点的个数 public static int getLeafNodesNoRecursion(TreeNode root){ if(root==null){ return 0; } int size=0; Deque<TreeNode> queue=new LinkedList<>(); queue.offer(root); while (!queue.isEmpty()){ TreeNode cur=queue.poll(); if(cur.left==null && cur.right==null){ size++; } if(cur.left!=null){ queue.offer(cur.left); } if(cur.right!=null){ queue.offer(cur.right); } } return size; } //层序遍历 public static void levelOrder(TreeNode root) { if(root==null){ return ; } // 借助队列来实现遍历过程 Deque<TreeNode> queue =new LinkedList<>(); queue.offer(root); while (!queue.isEmpty()){ int size=queue.size(); for (int i = 0; i < size; i++) { TreeNode cur=queue.poll(); System.out.print(cur.val+" "); if(cur.left!=null){ queue.offer(cur.left); } if(cur.right!=null){ queue.offer(cur.right); } } } } //传入一个以root为根节点的二叉树，就能求出该树的高度 public static int height(TreeNode root){ if(root==null){ return 0; } return 1+ Math.max(height(root.left),height(root.right)); } //求出以root为根节点的二叉树第k层的节点个数 public static int getKLevelNodes(TreeNode root,int k){ if(root==null || k<=0){ return 0; } if(k==1){ return 1; } return getKLevelNodes(root.left,k-1)+getKLevelNodes(root.right,k-1); } //判断当前以root为根节点的二叉树中是否包含指定元素val， //若存在返回true，不存在返回false public static boolean contains(TreeNode root,char value){ if(root==null){ return false; } if(root.val==value){ return true; } return contains(root.left,value) || contains(root.right,value); } public static void main(String[] args) { TreeNode root=build(); System.out.println("方法1(递归)：前序遍历的结果为:"); preOrder(root); System.out.println(); System.out.println("方法2(迭代)：前序遍历的结果为:"); preOrderNonRecursion(root); System.out.println(); System.out.println("方法1(递归)：中序遍历的结果为:"); inOrder(root); System.out.println(); System.out.println("方法2(迭代)：中序遍历的结果为:"); inorderTraversalNonRecursion(root); System.out.println(); System.out.println("方法1(递归)：后序遍历的结果为:"); postOrder(root); System.out.println(); System.out.println("方法2(迭代)：后序遍历的结果为:"); postOrderNonRecursion(root); System.out.println(); System.out.println(); System.out.println("层序遍历的结果为:"); levelOrder(root); System.out.println(); System.out.println(); System.out.println("方法1(递归)：当前二叉树一共有:"+getNodes(root)+"个节点数"); System.out.println("方法2(迭代)：当前二叉树一共有:"+getNodesNoRecursion(root)+"个节点数"); System.out.println("方法1(递归)：当前二叉树一共有:"+getLeafNodes(root)+"个叶子节点数"); System.out.println("方法2(迭代)：当前二叉树一共有:"+getLeafNodesNoRecursion(root)+"个叶子节点数"); System.out.println(contains(root,'E')); System.out.println(contains(root,'P')); System.out.println("当前二叉树的高度为:"+height(root)); System.out.println("当前二叉树第3层的节点个数为:"+getKLevelNodes(root,3)); } }
如上main引用结果如下：
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