111、257（遞歸，DFS）

類別：recursive, DFS

111-Minimum Depth of Binary Tree

題目描述：
Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
算法分析：
遞歸實現，對於根節點，如果不爲NULL, 則深度加1，分別在此深度的基礎上遞歸左子樹和右子樹，當左右子樹都爲NULL的時候，即爲最終的深度之一，判斷比較當前的深度並去較小值。
代碼實現：

class Solution {
public:
    int minDepth(TreeNode* root) {
        int result = INT_MAX;
        int len = 0;
        if (root == NULL) result = 0;
        else recursiveDepth(result, root, len + 1);
        return result;
    }
    void recursiveDepth(int& result, TreeNode* root, int len) {
        if (root->left == NULL && root->right == NULL) {
            result = min(result, len);
        }
        if (root->left != NULL) recursiveDepth(result, root->left, len + 1);
        if (root->right != NULL) recursiveDepth(result, root->right, len + 1);
    }
};

257-Binary Tree Paths

題目描述：

算法：與111的算法非常相似
代碼實現：

//  非常簡介巧妙的遞歸思路
vector<string> binaryTreePaths(TreeNode* root) {
    vector<string> result;
    if (root == NULL) return result;
    myBinaryTreePaths(result, TreeNode* root, to_string(root->val));
    return result;        
}
void myBinaryTreePaths(vector<string>& result, TreeNode* root, string str) {
    if (root->left == NULL && root->right == NULL) {
        result.push_back(str);
    }
    if (root->left != NULL) myBinaryTreePaths(result, root->left, str + "->" + to_string(root->left->val));
    if (root->right != NULL) myBinaryTreePaths(result, root->right, str + "->" + to_string(root->right->val));
}