111. Minimum Depth of Binary Tree

I was computing the depth from bottom to top which means you’ll have many redundant calls because you call h times from the root to get the depth and you’ll call h-1 times again from the left child to a leaf. Actually the h-1 times can be avoided if we compute the depth from top to bottom.

1:  /**  
2:   * Definition for a binary tree node.  
3:   * struct TreeNode {  
4:   *   int val;  
5:   *   TreeNode *left;  
6:   *   TreeNode *right;  
7:   *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
8:   * };  
9:   */  
10:  class Solution {  
11:  private:  
12:    int minD = INT_MAX;  
13:  public:  
14:    int minDepth(TreeNode* root) {  
15:      if (root == NULL) return 0;  
16:      helper(root, 1);  
17:      return minD;  
18:    }  
19:    void helper(TreeNode *root, int depth) {  
20:      if (root->left == NULL && root->right == NULL) { minD = min(minD, depth); return; }  
21:      if (root->left) helper(root->left, depth+1);  
22:      if (root->right) helper(root->right, depth+1);  
23:    }  
24:  };