二叉樹最小深度&根據遍歷結果構建二叉樹

leetcode_01

minimum-depth-of-binary-tree

求二叉樹的最小深度

class Solution
{
public:
    int run(TreeNode *root)
    {
        if(root == NULL)
        {
            return 0;
        }
        if(root->left == NULL && root->right == NULL)
        {
            return 1;
        }
        if(root->left == NULL)
        {
            return run(root->right) + 1;
        }
        if(root->right == NULL)
        {
            return run(root->left) + 1;
        }
        if(root->left != NULL && root->right != NULL)
        {
            int leftDepth = run(root->left) + 1;
            int rightDepth = run(root->right) + 1;
            return (leftDepth > rightDepth) ? rightDepth : leftDepth;
        }
        return 0;
    }
};

根據中序遍歷和後序遍歷結果構建二叉樹

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution
{
public:

    //在中序遍歷序列中找到根結點的位置
    int findRoot(vector<int> &array, int start, int end, int key)
    {
        for (int i = start; i <= end; i++)
        {
            if (array[i] == key)
            {
                return i;
            }
        }
        return -1;
    }

    TreeNode *build(vector<int> &inorder, int inStart, int inEnd, 
                    vector<int> &postorder, int postStart, int postEnd)
    {
        if (inStart > inEnd)
        {
            return NULL;
        }

        //根據後序遍歷序列的最後一個數字建立根結點root
        TreeNode *root = new TreeNode(postorder[postEnd]);

        //獲取根結點的位置position
        int position = findRoot(inorder, inStart, inEnd, postorder[postEnd]);
        //創建左子樹
        root->left = build(inorder, inStart, position - 1, 
                           postorder, postStart, 
                           postStart + (position - inStart - 1));
        //創建右子樹
        root->right = build(inorder, position + 1, inEnd, 
                            postorder, postStart + (position - inStart), 
                            postEnd - 1);

        return root;    //返回根結點
    }

    TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder)
    {
        if(inorder.size() != postorder.size())
            return NULL;
        return build(inorder, 0, inorder.size() - 1, 
                     postorder, 0, postorder.size() - 1);
    }
};