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題目:
https://leetcode.com/problems/minimum-depth-of-binary-tree/description/

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class Solution {
public:
    int minDepth(TreeNode* root) {
        if(root == nullptr) return 0; // empty tree
        if(root->left == nullptr && root->right == nullptr) return 1; // leaf node
        int ldepth = INF, rdepth = INF; // initially set to INF to prevent undefined value
        if(root->left != nullptr) ldepth = minDepth(root->left);
        if(root->right != nullptr) rdepth = minDepth(root->right);
        return min(ldepth, rdepth) + 1; // the smaller one plus myself
    }
};