111. Minimum Depth of Binary Tree

• Difficulty: Easy

• Topics: Tree, Depth-first Search, Breadth-first Search

• Similar Questions:

  • Binary Tree Level Order Traversal

  • Maximum Depth of Binary Tree

Problem:

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.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its minimum depth = 2.

Solutions:

/**
 * 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 minDepth(TreeNode* root) {
        if (root == nullptr)    return 0;
        int ret = INT_MAX;
        if (root->left) {
            ret = min(ret, 1 + minDepth(root->left));
        } 

        if (root->right) {
            ret = min(ret, 1 + minDepth(root->right));
        }

        return ret == INT_MAX ? 1 : ret;
    }
};