diff --git a/Week_04/id_39/LeetCode_309_39.js b/Week_04/id_39/LeetCode_309_39.js
new file mode 100644
index 00000000..1e81efbc
--- /dev/null
+++ b/Week_04/id_39/LeetCode_309_39.js
@@ -0,0 +1,15 @@
+/**
+ * @param {number[]} prices
+ * @return {number}
+ * https://leetcode.com/problems/best-time-to-buy-and-sell-stock-with-cooldown/
+ * 只考虑前一天
+ */
+var maxProfit = function(prices) {
+    let sell = [0], buy = [-prices[0]], cool = [0];
+    for (let i = 1; i < prices.length; i++) {
+        sell[i] = Math.max(buy[i-1] + prices[i], sell[i-1]);
+        buy[i] = Math.max(cool[i-1] - prices[i], buy[i-1]);
+        cool[i] = Math.max(sell[i-1], buy[i-1], cool[i-1]);
+    }
+    return sell[sell.length-1];
+};
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diff --git a/Week_04/id_39/LeetCode_70_39.js b/Week_04/id_39/LeetCode_70_39.js
new file mode 100644
index 00000000..aae5d9eb
--- /dev/null
+++ b/Week_04/id_39/LeetCode_70_39.js
@@ -0,0 +1,12 @@
+/**
+ * @param {number} n
+ * @return {number}
+ * https://leetcode.com/problems/climbing-stairs/
+ */
+var climbStairs = function(n) {
+    let dp = [0, 1, 2];
+    for (let i = 3; i <= n; i++) {
+        dp[i] = dp[i-1] + dp[i-2];
+    }
+    return dp[n];
+};
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diff --git a/Week_04/id_39/LeetCode_84_39.js b/Week_04/id_39/LeetCode_84_39.js
new file mode 100644
index 00000000..893e88cf
--- /dev/null
+++ b/Week_04/id_39/LeetCode_84_39.js
@@ -0,0 +1,35 @@
+/**
+ * @param {number[]} heights
+ * @return {number}
+ * https://leetcode.com/problems/largest-rectangle-in-histogram/
+ * 回顾第一周视频，以栈的解法解答该题目
+ */
+class newStack {
+    constructor () {
+        this.arr = [...arguments];
+    }
+    peek () {
+        return this.arr[this.arr.length - 1];
+    }
+    push (node) {
+        this.arr.push(node);
+    }
+    pop () {
+        return this.arr.pop();
+    }
+}
+
+var largestRectangleArea = function(heights) {
+    let stack = new newStack(-1), maxArea = 0;
+    
+    for (let i = 0; i < heights.length; i++) {
+        while(heights[i] < heights[stack.peek()] && stack.peek() != -1) {
+            maxArea = Math.max(maxArea, heights[stack.pop()] * (i - stack.peek() - 1 ));
+        }
+        stack.push(i);
+    }
+    while (stack.peek() != -1) {
+        maxArea = Math.max(maxArea, heights[stack.pop()] * (heights.length - stack.peek() - 1 ));
+    }
+    return maxArea;
+};
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diff --git a/Week_04/id_39/NOTE.md b/Week_04/id_39/NOTE.md
index 107ea7d6..dadb935a 100644
--- a/Week_04/id_39/NOTE.md
+++ b/Week_04/id_39/NOTE.md
@@ -1 +1,6 @@
 # 学习笔记
+
+- 这周把之前的用单调栈求解面积的视频又看了一遍，并做了代码实现，还是有点小迷糊，还是得多做
+- DP的题目
+    - 思路都是从结果开始考虑，找子问题，看能不能倒推回去
+    - 不过边界限制条件不太好想，得多写多练才可以
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