102. 二叉树的层序遍历

/**
 * 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 levelOrder = function(root) {
    const queue = [];
    if (root !== null) queue.push(root);
    const res = [];
    while (queue.length) {
        const row = [];
        let size = queue.length;
        while (size) {
            // 队列是从头出去
            const node = queue.shift();
            row.push(node.val);
            if (node.left) queue.push(node.left);
            if (node.right) queue.push(node.right);
            size--;
        }
        res.push(row);
    }
    return res;
};

107. 二叉树的层序遍历 II

/**
 * 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 levelOrderBottom = function(root) {
     const queue = [];
     const res = [];
     if (root !== null) queue.push(root);
     while (queue.length) {
        const row = [];
        let size = queue.length;
        while (size) {
            const node = queue.shift();
            row.push(node.val);
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
            size--;
        }
        res.unshift(row);
     }
     return res;
};

199. 二叉树的右视图

/**
 * 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 rightSideView = function(root) {
    const queue = [], res = [];
    if (root !== null) queue.push(root);
    while (queue.length) {
        let size = queue.length;
        while (size) {
            const node = queue.shift();
            // 每一层的最后一个元素
            if (size === 1) res.push(node.val);
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
            size--;
        }
    }
    return res;
};

637. 二叉树的层平均值

/**
 * 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 averageOfLevels = function(root) {
    const queue = [], res = [];
    if (root !== null) queue.push(root);
    while (queue.length) {
        let size = queue.length;
        // 处理每一层的每个元素
        let sum = 0;
        // 需要得到每一层的length,所以需要用for而非while
        for (let i = 0; i < size; i++) {
            const node = queue.shift();
            sum += node.val;
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
        }
        res.push(sum / size);
    }
    return res;
};

429. N 叉树的层序遍历

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

/**
 * @param {Node|null} root
 * @return {number[][]}
 */
var levelOrder = function(root) {
    const queue = [], res = [];
    if (root !== null) queue.push(root);
    while (queue.length) {
        const row = [];
        let size = queue.length;
        while (size) {
            const node = queue.shift();
            row.push(node.val);

            // node.children
            for (let child of node.children) {
                child && queue.push(child);
            }
            size--;
        }
        res.push(row);
    }
    return res;
};

515. 在每个树行中找最大值

/**
 * 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 largestValues = function(root) {
    const queue = [], res = [];
    if (root !== null) queue.push(root);
    while (queue.length) {
        let size = queue.length;
        let max = queue[0].val;
        while (size) {
            const node = queue.shift();
            max = Math.max(max, node.val);
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
            size--;
        }
        res.push(max);
    }
    return res;
};

116. 填充每个节点的下一个右侧节点指针

/**
 * // Definition for a Node.
 * function Node(val, left, right, next) {
 *    this.val = val === undefined ? null : val;
 *    this.left = left === undefined ? null : left;
 *    this.right = right === undefined ? null : right;
 *    this.next = next === undefined ? null : next;
 * };
 */

/**
 * @param {Node} root
 * @return {Node}
 */
var connect = function(root) {
    const queue = [];
    if (root !== null) queue.push(root);
    while (queue.length) {
        let size = queue.length;
        for (let i = 0; i < size; i++) {
            const node = queue.shift();
            // 【关键代码】每一层除了最后一个
            if (i < size - 1) node. next = queue[0];
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
        }   
    }
    return root;
    
};

117. 填充每个节点的下一个右侧节点指针 II

/**
 * // Definition for a Node.
 * function Node(val, left, right, next) {
 *    this.val = val === undefined ? null : val;
 *    this.left = left === undefined ? null : left;
 *    this.right = right === undefined ? null : right;
 *    this.next = next === undefined ? null : next;
 * };
 */

/**
 * @param {Node} root
 * @return {Node}
 */
var connect = function(root) {
    const queue = [];
    if (root !== null) queue.push(root);
    while (queue.length) {
        let size = queue.length;
        for (let i = 0; i < size; i++) {
            const node = queue.shift();
            if (i < size - 1) node.next = queue[0];
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
        }
    }
    return root;
};

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) {
    const queue = [];
    let height = 0;
    if (root !== null) queue.push(root);
    while (queue.length) {
        let size = queue.length;
        while (size) {
            const node = queue.shift();
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
            size--;
        }
        height++;
    }
    return height;
};

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) {
    // 只有当左右孩子都为空时，才停止遍历，返回 height
    const queue = [];
    let height = 0;
    if (root !== null) queue.push(root);
    while (queue.length) {
        let size = queue.length;
        // 记录最小深度
        height++;
        while (size) {
            const node = queue.shift();
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
            if (!node.left && !node.right) return height;
            size--;
        }
    }
    return height;
};

226. 翻转二叉树

/**
 * 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 {TreeNode}
 */
var invertTree = function(root) {
    if (root === null) return root;
    
    const temp = root.left;
    root.left = root.right;
    root.right = temp;

    invertTree(root.left);
    invertTree(root.right);
    return root;
};

101. 对称二叉树

/**
 * 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 {boolean}
 */
var isSymmetric = function(root) {
    // 比较函数
    // 左右共5种情况
    const compare = function(left, right) {
        if (left !== null && right === null) return false;
        else if (left === null && right !== null) return false;
        else if (left === null && right === null) return true;
        else if (left.val !== right.val) return false;

        let outside = compare(left.left, right.right);
        let inside = compare(left.right, right.left);
        return outside && inside;
    }

    if (root === null) return true;
    return compare(root.left, root.right);
};