［LeetCode］Binary Tree Right Side View

解题思路：

宽度搜索，记录每一个node的height，当遇到当前node的height与前一个node的height不一样时，说明前一个node是那个height level的最右边的node，可以把val加入return vector

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<int> rightSideView(TreeNode* root) {
if ( root == NULL ) return {};

queue<TreeNode* > st;
queue<int> stHeight;
int preHeight = 0;
TreeNode* preNode = root;

vector<int> ret;

st.push(root);
stHeight.push(0);

while(!st.empty()){
TreeNode* curNode = st.front(); st.pop();
int curHeight = stHeight.front(); stHeight.pop();
if (curNode != NULL){
if (curHeight != preHeight){
ret.push_back(preNode->val);
}
preNode = curNode;
preHeight = curHeight;

st.push(curNode->left);
stHeight.push(curHeight+1);
st.push(curNode->right);
stHeight.push(curHeight+1);
}
}

if (preNode != NULL){
ret.push_back(preNode->val);
}
return ret;
}
};