二叉树

二叉树的定义不多赘述，先贴上二叉树的代码

public class TreeNode {
    int val;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        // TODO Auto-generated constructor stub
        this.val = val;
    }
}

前序遍历

　　递归实现：

public void preOrderRecur(TreeNode root){
    if(root == null)
        return root;
    System.out.print(head.value+" ");
    preOrderRecur(head.left);
    preOrderRecur(head.right);
}

　　非递归实现：

public void preOrderStack(TreeNode root){
    if(root == null)
        return root;
    Stack<TreeNode> stack = new Stack<>();
    stack.push(root);
    TreeNode cur = null;
    while(!stack.isEmpty()){
        cur = stack.pop();
        System.out.print(cur.val+" ");
        if(cur.right != null)
            stack.push(cur.right);
        if(cur.left != null)
            stack.push(cur.left);        
    }
}

中序遍历

　　递归实现：

public void inOrderRecur(TreeNode root){
    if(root == null)
        return root;
    inOrderRecur(root.left);
    System.out.print(root.value+" ");
    inOrderRecur(root.right);    
}

　　非递归实现：

public void inOrderStack(TreeNode root){
    if(root == null)
        return root;
    Stack<TreeNode> stack = new Stack<>();
    TreeNode cur = root;
    TreeNode node = null;
    while(cur != null){
        stack.push(cur);
        cur = cur.left;
        if(cur == null){
            node = stack.pop();
            System.out.print(node.val+" ");
            cur = node.right;
        }
    }
    
}

后序遍历

　　递归实现：

public void posOrderRecur(TreeNode root){
    if(root == null)
        return root;
    posOrderRecur(root.left);
    posOrderRecur(root.right);
    System.out.print(root.value+" ");    
}

　　非递归实现：

public void posOrderStack(TreeNode root){
    if(root == null)
        return root;
    Stack<TreeNode> stack1 = new TreeNode<>();
    Stack<TreeNode> stack2 = new TreeNode<>();
    TreeNode cur = root;
    stack1.push(root);
    while(!s1.isEmpty()){
        cur = s1.pop();
        stack2.push(cur); 
        if(cur.left != null)
            stack1.push(cur.left);
        if(cur.right != null)        
            stack1.push(cur.right);
    }    
    while(stack2.hasNext())
        System.out.print(stack2.pop().val+" ");
}

注：不论是递归还是非递归方法，遍历整棵树的时间复杂度都是O(N),额外的空间复杂度都是O(L),N为二叉树的节点数，L为二叉树的层数。