104. 二叉树的最大深度

递归

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
//  递归法
var maxDepth = function(root) {
    
    const getHeight = function(node) {
        if (node === null) return 0;
        let leftHeight = getHeight(node.left);
        let rightHeight = getHeight(node.right);
        let maxHeight = 1 + Math.max(leftHeight, rightHeight);
        return maxHeight;
    }

    return getHeight(root);

111. 二叉树的最小深度

下面这样写会超时

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var minDepth = function(root) {
    const getMinHeight = function(node) {
        if (node === null) return 0;
        let leftHeight = getMinHeight(node.left);
        let rightHeight = getMinHeight(node.right);

        if (!node.left && node.right) return 1 + getMinHeight(node.right);
        if (!node.right && node.left) return 1 + getMinHeight(node.left);
        let minHeight = 1 + Math.min(leftHeight, rightHeight);
        return minHeight;
    }

    return getMinHeight(root);
};  

以下可以运行

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var minDepth = function(root) {
    if (!root) return 0;
    if (!root.left && !root.right) return 1;
    if (!root.left) return 1 + minDepth(root.right);
    if (!root.right) return 1 + minDepth(root.left);
    
    let leftDepth = minDepth(root.left);
    let rightDepth = minDepth(root.right);
    let min = 1 + Math.min(leftDepth, rightDepth);
    return min;
};

222. 完全二叉树的节点个数

递归

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
//  递归
var countNodes = function(root) {
    if (!root) return 0;
    let leftNum = countNodes(root.left);
    let rightNum = countNodes(root.right);
    let sumNum = leftNum + rightNum + 1;
    return sumNum;
};

完全二叉树性质

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var countNodes = function(root) {
    if (!root) return 0;
    let left = root.left;
    let right = root.right;
    let leftHeight = 0, rightHeight = 0;
    while (left) {
        left = left.left;
        leftHeight++;
    }
    while (right) {
        right = right.right;
        rightHeight++;
    }
    if (leftHeight === rightHeight) {
        return Math.pow(2, leftHeight+1) - 1
    }

    let leftNum = countNodes(root.left);
    let rightNum = countNodes(root.right);
    let sumNum = leftNum + rightNum + 1;
    return sumNum;
};