学步园

Binary
Tree Level Order Traversal II:

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},

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7]
  [9,20],
  [3],
]

跟前面那个没什么区别，你要顺序添加了做一个reverse也行。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
    	if ( !root )
			return vector<vector<int> >();

		deque<vector<int> > ans;
		vector<int> level;
		queue<TreeNode* > Q;
		Q.push(root);
		int lsum=1;
		while(lsum)
		{
			level.clear();
			int next=0;
			while(lsum--)
			{
				TreeNode* t=Q.front();
				Q.pop();
				level.push_back(t->val);
				if ( t->left )
				{
					Q.push(t->left);
					next++;
				}
				if ( t->right)
				{
					Q.push(t->right);
					next++;
				}
			}
			ans.push_front(level);
			lsum=next;
		}
		return vector<vector<int> >(ans.begin(),ans.end());
    }
};