[LeetCode]144. Binary Tree Preorder Traversal&94. Binary Tree Inorder Traversal

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本文详细介绍了二叉树的前序、中序遍历算法，包括递归、栈和莫里斯三种实现方式，并提供了具体的Java代码示例。

144 . Binary Tree Preorder Traversal
Medium

递归：

public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> results = new ArrayList<>();
    preorderTraversal(root, results);
    return results;
}

private void preorderTraversal(TreeNode root, List<Integer> results) {
    if (root == null)  return;
    results.add(root.val);
    preorderTraversal(root.left, results);
    preorderTraversal(root.right, results);
}

栈：

 public List<Integer> preorderTraversal(TreeNode root){
    List<Integer> res = new ArrayList<Integer>();
    if(root==null) return res;
    Stack<TreeNode> stack = new Stack<TreeNode>();
    TreeNode tn = root;
    while(tn!=null||!stack.isEmpty()){
        if(tn!=null){
            res.add(tn.val);
            if(tn.right!=null)
                stack.push(tn.right);
            tn = tn.left;
        }else{
            tn = stack.pop();
        }
    }
    return res;
}

莫里斯：

public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> results = new ArrayList<>();
    TreeNode crt = root;
    while (crt != null) {
        if (crt.left == null) {
            results.add(crt.val);
            crt = crt.right;
        } else {
            TreeNode temp = crt.left;
            while (temp.right != null && temp.right != crt) 
                temp = temp.right;
            if (temp.right == null) {
                results.add(crt.val);
                temp.right = crt;
                crt = crt.left;
            } else {
                temp.right = null;
                crt = crt.right;
            }
        }
    }
    return results;
}

94 . Binary Tree Inorder Traversal
Medium

递归：

public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> results = new ArrayList<>();
    inorderTraversal(root, results);
    return results;
}

private void inorderTraversal(TreeNode root, List<Integer> results) {
    if (root == null) return;
    inorderTraversal(root.left, results);
    results.add(root.val);
    inorderTraversal(root.right, results);
}

栈：

public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> results = new ArrayList<>();
    ArrayDeque<TreeNode> stack = new ArrayDeque<>();
    TreeNode crt = root;
    while (!stack.isEmpty() || crt != null) {
        if (crt != null) {
            stack.push(crt);
            crt = crt.left;
        } else {
            crt = stack.pop();
            results.add(crt.val);
            crt = crt.right;
        }
    }
    return results;
}

莫里斯：

public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> results = new ArrayList<>();
    TreeNode crt = root;
    while (crt != null) {
        if (crt.left == null) {
            results.add(crt.val);
            crt = crt.right;
        } else {
            TreeNode temp = crt.left;
            while (temp.right != null && temp.right != crt) 
                temp = temp.right;
            if (temp.right == null) {
                temp.right = crt;
                crt = crt.left;
            } else {
                temp.right = null;
                results.add(crt.val);
                crt = crt.right;
            }
        }
    }
    return results;
}