代码随想录算法训练营第十六天 | 104. 二叉树的最大深度 559. N 叉树的最大深度 111. 二叉树的最小深度 222. 完全二叉树的节点个数

104. 二叉树的最大深度

/**
 * 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 maxDepth = function(root) {
    let res = 0;

    if(!root) return res;

    let queue = [root];
    while(queue.length) {
        let len = queue.length;
        res++;
        for (let i = 0; i < len; i++) {
            let top = queue.shift();
            if (top.left) queue.push(top.left)
            if (top.right) queue.push(top.right)
        }
    }

    return res;
};

559. N 叉树的最大深度

/**
 * // Definition for a Node.
 * function Node(val,children) {
 *    this.val = val;
 *    this.children = children;
 * };
 */

/**
 * @param {Node|null} root
 * @return {number}
 */
var maxDepth = function(root) {
    let res = 0;

    if (!root) return res;

    let queue = [root];
    while(queue.length) {
        let len = queue.length;
        res++;
        for (let i = 0; i < len; i++) {
            let top = queue.shift();
            for (let j = 0; j < top.children.length; j++) {
                queue.push(top.children[j])
            }
        }
    }

    return res;
};

111. 二叉树的最小深度

/**
 * 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) {
    let res = 0;
    if (!root) return res;

    let queue = [root];
    while(queue.length) {
        let len = queue.length;
        res++;
        for (let i = 0; i < len; i++) {
            let top = queue.shift();
            if (top.left) queue.push(top.left);
            if (top.right) queue.push(top.right);
            if (!top.left && !top.right) return res;
        }
    }

    return res;
};

222. 完全二叉树的节点个数

迭代：

/**
 * 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) {
    let res = 0;
    if (!root) return res;

    let queue = [root];
    while(queue.length) {
        let len = queue.length;
        for (let i = 0; i < len; i++) {
            let top = queue.shift();
            res++;
            if (top.left) queue.push(top.left);
            if (top.right) queue.push(top.right);
        }
    }

    return res;
};

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}
 */
var countNodes = function(root) {
    function getNum(node) {
        if (!node) return null;
        let leftNum = getNum(node.left);
        let rightNum = getNum(node.right);
        let total = leftNum + rightNum + 1;
        return total;
    }

    return getNum(root)
};

利用完全二叉树性质：

/**
 * 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 leftDepth = 0;
    let rightDepth = 0;

    let left = root.left;
    let right = root.right;

    while(left) {
        left = left.left;
        leftDepth++;
    }

    while(right) {
        right = right.right;
        rightDepth++;
    }

    if (leftDepth === rightDepth) {
        return Math.pow(2, leftDepth + 1) - 1; // 注意这里要+1
    }

    return countNodes(root.left) + countNodes(root.right) + 1;
};