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[LintCode/LeetCode] Binary Tree Zigzag Level Orde

AlphaGooo / 2733人閱讀

Problem

Given a binary tree, return the zigzag level order traversal of its nodes" values. (ie, from left to right, then right to left for the next level and alternate between).

Example

Given binary tree {3,9,20,#,#,15,7},

    3
   / 
  9  20
    /  
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]
Note Solution 
public class Solution {
    public ArrayList> zigzagLevelOrder(TreeNode root) {
        ArrayList> res = new ArrayList();
        if (root == null) return res;
        boolean LR = true;
        Stack stack = new Stack();
        stack.push(root);
        while (!stack.isEmpty()) {
            Stack curStack = new Stack();
            ArrayList curList = new ArrayList();
            while (!stack.isEmpty()) {
                TreeNode cur = stack.pop();
                curList.add(cur.val);
                if (LR) {
                    if (cur.left != null) curStack.push(cur.left);
                    if (cur.right != null) curStack.push(cur.right);
                }
                else {
                    if (cur.right != null) curStack.push(cur.right);
                    if (cur.left != null) curStack.push(cur.left);
                }
            }
            stack = curStack;
            res.add(curList);
            LR = !LR;
        }
        return res;
    }
}

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