feat: 从上到下打印二叉树 III

工具/二叉树.js

@@ -1,9 +1,9 @@
 class TreeNode {
-    constructor(val) {
-        this.val = val
-        this.left = null
-        this.right = null
-    }
+  constructor(val) {
+    this.val = val
+    this.left = null
+    this.right = null
+  }
 }
 
 module.exports = TreeNode

栈和队列/二叉树的前序遍历-144.js

@@ -0,0 +1,47 @@
+const TreeNode = require("../工具/二叉树")
+
+/**
+ * @param {TreeNode} root
+ * @return {number[]}
+ */
+let preorderTraversal = function (root) {
+  let res = []
+  let stack = [
+    {
+      type: "go",
+      node: root,
+    },
+  ]
+
+  while (stack.length) {
+    let { type, node } = stack.pop()
+
+    if (!node) continue
+
+    if (type === "print") {
+      res.push(node.val)
+    }
+
+    if (type === "go") {
+      stack.push({ type: "print", node })
+
+      if (node.right) {
+        stack.push({ type: "go", node: node.right })
+      }
+
+      if (node.left) {
+        stack.push({ type: "go", node: node.left })
+      }
+    }
+  }
+
+  return res
+}
+
+const tree = new TreeNode(1)
+tree.left = new TreeNode(4)
+tree.left.left = new TreeNode(5)
+tree.right = new TreeNode(6)
+tree.right.right = new TreeNode(7)
+
+console.log(preorderTraversal(tree))

栈和队列/从上到下打印二叉树III-面试题32.js

@@ -0,0 +1,39 @@
+const TreeNode = require("../工具/二叉树")
+
+let levelOrder = function (root) {
+  let queue = [root]
+  let res = []
+  if (!root) return res
+  let level = 0
+  while (queue.length) {
+    level++
+    let subRes = []
+    let len = queue.length
+    let shouldReverse = level % 2 === 0
+
+    for (let i = 0; i < len; i++) {
+      let node = queue.shift()
+      subRes.push(node.val)
+      if (node.left) {
+        queue.push(node.left)
+      }
+      if (node.right) {
+        queue.push(node.right)
+      }
+    }
+    // 偶数行 把结果子数组reverse即可
+    if (shouldReverse) {
+      subRes.reverse()
+    }
+    res.push(subRes)
+  }
+  return res
+}
+
+let tree = new TreeNode(1)
+tree.left = new TreeNode(2)
+tree.left.left = new TreeNode(4)
+tree.right = new TreeNode(3)
+tree.right.right = new TreeNode(5)
+
+console.log(levelOrder(tree))