2019-3-27

const canvas = document.querySelector('#draw');
const c = canvas.getContext('2d');
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
c.lineJoin = 'round';
c.lineCap = 'round';
c.lineWidth = 100;
let isDrawing = false;
let lastX = 0;
let lastY = 0;
let hue = 0;
function draw(e) {
  if (!isDrawing) return;
  c.strokeStyle = `hsl(${hue}, 50%, 50%)`;
  c.beginPath();
  c.moveTo(lastX, lastY);
  c.lineTo(e.offsetX, e.offsetY);
  c.stroke();
  [lastX, lastY] = [e.offsetX, e.offsetY];
  hue++;
}
canvas.addEventListener('mousemove', draw);
canvas.addEventListener('mousedown', (e) => {
  [lastX, lastY] = [e.offsetX, e.offsetY];
  isDrawing = true
});
canvas.addEventListener('mouseup', () => {isDrawing = false});

不会修改原数组！！

So:
Use if (arr.length) {}

class Node {
  constructor(data) {
    this.data = data;
    this.children = [];
  }
  add(data) {
    this.children.push(new Node(data))
  }
  remove(data) {
    this.children = this.children.filter((child) => {
      return child.data !== data;
    })
  }
}
class Tree {
  constructor() {
    this.root = null;
  }
  traverseBF(fn) {
    const list = [this.root];
    while (list.length) {
      const node = list.shift();
      list.push(...node.children);
      fn(node);
    }
  }
  traverseDF(fn) {
    const list = [this.root];
    while (list.length) {
      const node = list.shift();
      list.unshift(...node.children);
      fn(node);
    }
  }
}

class Node {
  constructor(data) {
    this.data = data;
    this.left = null;
    this.right = null;
  }
  // insert 函数不需要 return 什么东西，可以直接回调
  insert(data) {
    if (this.left && data < this.data) {
       this.left.insert(data);
    } else if (data < this.data) {
      this.left = (new Node(data));
    }
    if (this.right && data > this.data) {
      this.right.insert(data);
    } else if (data > this.data) {
      this.right = (new Node(data));
    }
  }
  // contain 函数需要 return. 所以回调中要一直 return
  contains(data) {
    if (data === this.data) {
      return this;
    } else if (this.left && data < this.data) {
      return this.left.contains(data);
    } else if (this.right && data > this.data) {
      return this.right.contains(data);
    } else {
      return null;
    }
  }
}