513 Find Bottom Left Tree Value

bfs：
/**
 * 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:
    int findBottomLeftValue(TreeNode* root) {
        int ans;
        queue<TreeNode*>q;
        queue<int>level;
        q.push(root);
        level.push(0);
        int x=-1;
        while(q.size()){
            TreeNode*node=q.front();
            q.pop();
            int lev=level.front();
            level.pop();
            if(node->left!=NULL){
                q.push(node->left);
                level.push(lev+1);
            }
            if(node->right!=NULL){
                q.push(node->right);
                level.push(lev+1);
            }
            if(lev>x){
                x=lev;
                ans=node->val;
            }
        }
        return ans;
    }
};
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递归 from discuss
/**
 * 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:
    int findBottomLeftValue(TreeNode* root) {
        int max_depth=0,depth=0;
        int leftmost_val=root->val;
        find(root,max_depth,depth,leftmost_val);
        return leftmost_val;
    }
    void find(TreeNode*node,int&maxdepth,int depth,int&leftmostval){
        //maxdepth是对max_depth的引用，每次depth大于maxdepth都会修改maxdepth的值
        if(node==NULL)return;
        find(node->left,maxdepth,depth+1,leftmostval);
        find(node->right,maxdepth,depth+1,leftmostval);
        if(depth>maxdepth){//该判断只在n+1层的最左端节点为true
            maxdepth=depth;
            leftmostval=node->val;
        }
        
    }
};