从底向上层次遍历二叉树
题目原型：
Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
For example:
Given binary tree {3,9,20,#,#,15,7},
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3 / \ 9 20 / \ 15 7 |
return its bottom-up level order traversal as:
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[ [15,7] [9,20], [3], ] |
基本思路：
由于是从底向上，所以用到了栈。
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public ArrayList> levelOrderBottom(TreeNode root) { Stack> stack = new Stack>(); ArrayList> list = new ArrayList>(); ArrayList<TREENODE> nodeSet = new ArrayList<TREENODE>(); ArrayList<TREENODE> tmp ; ArrayList<INTEGER> numSet ; if(root!=null) { nodeSet.add(root); while(nodeSet.size()>0) { tmp = new ArrayList<TREENODE>(); numSet = new ArrayList<INTEGER>(); //添加到stack中 for(TreeNode tn : nodeSet) numSet.add(tn.val); //添加到stack中 stack.push(numSet); //求下一层的节点 for(TreeNode it : nodeSet) { if(it.left!=null) tmp.add(it.left); if(it.right!=null) tmp.add(it.right); } nodeSet = tmp; } //添加到list中 while(stack.size()>0) { ArrayList<INTEGER> rs = stack.pop(); list.add(rs); } } return list; } |
