1、利用遞歸的方式獲取樹(shù)的前序遍歷結(jié)果

 //獲取樹(shù)的前序遍歷
public static String preTraver(TreeNode root){
    if (root!=null) {
        re += root.val;
        if (root.left != null)
            preTraver(root.left);
        if (root.right != null)
            preTraver(root.right);
    }
    return re;
}

2、 利用遞歸的方式獲取樹(shù)的中序遍歷結(jié)果

    //獲取樹(shù)的中序遍歷
public static String medTraver(TreeNode root){
    if (root!=null) {
        if (root.left != null)
            medTraver(root.left);
        re += root.val;
        if (root.right != null)
            medTraver(root.right);
    }
    return re;
}

3、利用遞歸的方式獲取樹(shù)的后序遍歷結(jié)果

//獲取樹(shù)的后序遍歷
public static String aftTraver(TreeNode root){
    if (root!=null) {
        if (root.left != null)
            preTraver(root.left);
        if (root.right != null)
            preTraver(root.right);
        re += root.val;
    }
    return re;
}

4、利用隊(duì)列實(shí)現(xiàn)層次遍歷

//獲取樹(shù)的層次遍歷
public static String levelTraver(TreeNode root){
    Queue<TreeNode> queue = new ArrayDeque<TreeNode>();
    TreeNode node;
 if (root!=null)
     queue.offer(root);
 while (!queue.isEmpty()){
     node=queue.poll();
     re+=node.val;
     if (node.left!=null)
         queue.offer(node.left);
     if (node.right!=null)
         queue.offer(node.right);
 }
    return re;
}

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