04月
01

二叉树前序遍历

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二叉树前序遍历

二叉树前序遍历：首先访问根节点，然后遍历左子树，最后遍历右子树
二叉树前序遍历

递归实现前序遍历(Java)

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        pre(root,result);
        return result;
    }
    
    public void pre(TreeNode root, List<Integer> result){
        if(root == null){
            return;
        }
        result.add(root.val);
        pre(root.left,result);
        pre(root.right,result);
    }
}

堆栈迭代实现前序遍历(Java)

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        if(root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode treeNode = stack.pop();
            result.add(treeNode.val);
            if(treeNode.right != null){
                stack.push(treeNode.right);
            }
            if(treeNode.left != null){
                stack.push(treeNode.left);
            }
        }
        return result;
    }
}
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