数据结构奇妙旅程之二叉树初阶

class Node {
int value; // 树中存储的数据
Node firstChild; // 第一个孩子引用
Node nextBrother; // 下一个兄弟引用
}

public class BinaryTree {
    static class TreeNode {
        public char val;
        public TreeNode left;
        public TreeNode right;
        public TreeNode(char val) {
            this.val = val;
        }
    }
 
    //以穷举的方式 创建一棵二叉树出来
    public TreeNode createTree() {
        TreeNode A = new TreeNode('A');
        TreeNode B = new TreeNode('B');
        TreeNode C = new TreeNode('C');
        TreeNode D = new TreeNode('D');
        TreeNode E = new TreeNode('E');
        TreeNode F = new TreeNode('F');
        TreeNode G = new TreeNode('G');
        TreeNode H = new TreeNode('H');
        A.left = B;
        A.right = C;
        B.left = D;
        B.right = E;
        C.left = F;
        C.right = G;
        E.right = H;
        return A;
    }
    //前序遍历
   public void preOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        System.out.print(root.val + " ");
        preOrder(root.left);
        preOrder(root.right);
    }
    //中序遍历
    public void inOrder(TreeNode root) {
        if(root == null){
            return;
        }
        inOrder(root.left);
        System.out.print(root.val + " ");
        inOrder(root.right);
    }
    //后序遍历
    public void postOrder(TreeNode root) {
        if(root == null){
            return;
        }
        postOrder(root.left);
        postOrder(root.right);
        System.out.print(root.val + " ");
    }
    // 获取二叉树中节点的个数
    public int size(TreeNode root) {
        if(root == null) {
            return 0;
        }
        return size(root.left)+size(root.right)+1;
    }
    // 获取叶子节点的个数
    public int getLeafNodeCount(TreeNode root) {
        if(root == null) {
            return 0;
        }
        if(root.left == null && root.right == null) {
            return 1;
        }
        return getLeafNodeCount(root.left) + getLeafNodeCount(root.right);
    }
    // 获取第K层节点的个数
    public int getKLevelNodeCount(TreeNode root,int k) {
        if(root == null) {
            return 0;
        }
        if(k == 1) {
            return 1;
        }
        return getKLevelNodeCount(root.left,k-1) + getKLevelNodeCount(root.right,k-1);
    }
    // 获取二叉树的高度
    public int getHeight(TreeNode root) {
      if(root == null) {
          return 0;
      }
      int leftH = getHeight(root.left);
      int rightH = getHeight(root.right);
      return Math.max(leftH,rightH)+1;
    }
    // 检测值为value的元素是否存在
    public boolean find(TreeNode root,char val) {
        if(root == null) {
            return false;
        }
        if(root.val == val) {
            return true;
        }
        return find(root.left, val) || find(root.right, val);
    }
    //层序遍历使用队列来辅助
    //当涉及到层序遍历时，通常情况下使用队列来实现会更为简单和高效
    public void levelOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while (!q.isEmpty()) {
           TreeNode cur = q.poll();
            System.out.print(cur.val + " ");
            if(cur.left != null) {
                q.offer(cur.left);
            }
            if(cur.right != null) {
                q.offer(cur.right);
            }
        }
    }
    // 判断一棵树是不是完全二叉树
    public boolean isCompleteTree(TreeNode root) {
        if (root == null) {
            return true;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean end = false;
        while (!queue.isEmpty()) {
            TreeNode current = queue.poll();
            if (current == null) {
                end = true;
            } else {
                if (end) {
                    return false; // 如果已经遇到空节点，再遇到非空节点，说明不是完全二叉树
                }
                queue.offer(current.left);
                queue.offer(current.right);
            }
        }
        return true;
    }
}