LeetCode

Minimum Depth of Binary Tree

2014/1/8 04:29

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.

Solution:

　　The height of a tree is defined as the longest path from the root node down to the farthest leaf node. Here in this problem it's the opposite.

　　The code below is simple enough to act as explanation for itself. An extra global variable is used to record the minimum depth during the recursive calls.

　　Time complexity is O(n), where n is the number of nodes in the tree. Space complexity is O(1).

Accepted code:

// 1WA, 1AC, good~
/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    // 1WA, only depths of leaf nodes count
    int minDepth(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(root == nullptr){
            return 0;
        }
        
        result = INT_MAX;
        _minDepth(root, 1);
        return result;
    }
private:
    int result;
    
    const int &mymin(const int &x, const int &y) {
        return (x < y ? x : y);
    }
    
    int _minDepth(TreeNode *root, int depth) {
        if(root->left == nullptr && root->right == nullptr){
            if(depth < result){
                result = depth;
            }
        }
        
        if(root->left != nullptr){
            _minDepth(root->left, depth + 1);
        }
        if(root->right != nullptr){
            _minDepth(root->right, depth + 1);
        }
    }
};