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xutao · xutao · commit 80b138d45cca · 2020-07-06T00:36:07.000+08:00
diff --git a/Week04/LemonadeChange.java b/Week04/LemonadeChange.java
@@ -0,0 +1,35 @@
+/**
+ * 860
+ */
+class LemonadeChange {
+    public boolean lemonadeChange(int[] bills) {
+        int five = 0;
+        int ten = 0;
+        if (bills.length == 0) {
+            return false;
+        }
+        for (int i = 0; i < bills.length; i++) {
+            if (bills[i] == 5) {
+                five++;
+            } else if (bills[i] == 10) {
+                if (five == 0) {
+                    return false;
+                }
+                five--;
+                ten++;
+            } else {
+                if (ten > 0 && five > 0) {
+                    ten--;
+                    five--;
+                } else {
+                    if (five > 2) {
+                        five -= 3;
+                    } else {
+                        return false;
+                    }
+                }
+            }
+        }
+        return true;
+    }
+}
diff --git a/Week04/NOTE.md b/Week04/NOTE.md
@@ -1 +1,33 @@
-学习笔记
+```java
+class Solution {
+    public int findDisorder(int nums[]) throws Exception {
+        if (nums.length < 2) {
+            throw new Exception("error");
+        }
+        int left = 0;
+        int right = nums.length - 1;
+        int mid = (left + right) / 2;
+        while (left < right) {
+            if (nums[mid] == 0 && nums[mid] > nums[mid + 1]) {
+                return nums[mid];
+            }
+            if (nums[mid] == nums.length && nums[mid] < nums[mid - 1]) {
+                return nums[mid];
+            }
+            if (nums[left] < nums[mid]) {
+                right = mid;
+            } else {
+                left = mid;
+            }
+        }
+        throw new Exception("error");
+    }
+}
+```
+
+
+### 总结：
+- 1.贪心算法，只适用特定场景，有些切入点有点奇怪，需要自己理解。
+- 2.牛顿迭代法还是看不懂数学公式，还是套代码模板把。
+- 3.关于BFS和DFS还是需要多联系。
+- 4.二分查找有时候有边界值和数组越界的问题，需要在代码里面多注意
diff --git a/Week04/NumberOfIslands.java b/Week04/NumberOfIslands.java
@@ -0,0 +1,41 @@
+/**
+ * @author @sailength on 2020/7/6.
+ *         200
+ */
+public class NumberOfIslands {
+    public int numIslands(char[][] grid) {
+
+        if (grid.length == 0 || grid == null) {
+            return 0;
+        }
+        int lengthy = grid.length;
+        int lengthx = grid[0].length;
+
+        int num = 0;
+        for (int i = 0; i < lengthy; i++) {
+            for (int j = 0; j < lengthx; j++) {
+                if (grid[i][j] == '1') {
+                    num++;
+                    dfs(grid, i, j);
+                }
+            }
+        }
+        return num;
+    }
+
+    private void dfs(char[][] grid, int x, int y) {
+        if (grid.length == 0 || grid == null) {
+            return;
+        }
+        int lengthy = grid.length;
+        int lengthx = grid[0].length;
+        if (x < 0 || y < 0 || x >= lengthx || y >= lengthy || grid[x][y] == '0') {
+            return;
+        }
+        grid[x][y] = '0';
+        dfs(grid, x - 1, y);
+        dfs(grid, x + 1, y);
+        dfs(grid, x, y - 1);
+        dfs(grid, x, y + 1);
+    }
+}
