Merge pull request youngyangyang04#2509 from carlvine500/master

Update 0236.二叉树的最近公共祖先.md
andyliao5555 · May 4, 2024 · 825b36d · 825b36d
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diff --git a/problems/0037.解数独.md b/problems/0037.解数独.md
@@ -224,7 +224,7 @@ public:
 
 
 ### Java
-
+解法一:
 ```java
 class Solution {
     public void solveSudoku(char[][] board) {
@@ -291,7 +291,73 @@ class Solution {
     }
 }
 ```
+解法二(bitmap标记)
+```
+class Solution{
+    int[] rowBit = new int[9];
+    int[] colBit = new int[9];
+    int[] square9Bit = new int[9];
+
+    public void solveSudoku(char[][] board) {
+        // 1 10 11
+        for (int y = 0; y < board.length; y++) {
+            for (int x = 0; x < board[y].length; x++) {
+                int numBit = 1 << (board[y][x] - '1');
+                rowBit[y] ^= numBit;
+                colBit[x] ^= numBit;
+                square9Bit[(y / 3) * 3 + x / 3] ^= numBit;
+            }
+        }
+        backtrack(board, 0);
+    }
+
+    public boolean backtrack(char[][] board, int n) {
+        if (n >= 81) {
+            return true;
+        }
+
+        // 快速算出行列编号 n/9 n%9
+        int row = n / 9;
+        int col = n % 9;
+
+        if (board[row][col] != '.') {
+            return backtrack(board, n + 1);
+        }
+
+        for (char c = '1'; c <= '9'; c++) {
+            int numBit = 1 << (c - '1');
+            if (!isValid(numBit, row, col)) continue;
+            {
+                board[row][col] = c;    // 当前的数字放入到数组之中，
+                rowBit[row] ^= numBit; // 第一行rowBit[0],第一个元素eg: 1 , 0^1=1,第一个元素:4, 100^1=101,...
+                colBit[col] ^= numBit;
+                square9Bit[(row / 3) * 3 + col / 3] ^= numBit;
+            }
+            if (backtrack(board, n + 1)) return true;
+            {
+                board[row][col] = '.';    // 不满足条件，回退成'.'
+                rowBit[row] &= ~numBit; // 第一行rowBit[0],第一个元素eg: 1 , 101&=~1==>101&111111110==>100
+                colBit[col] &= ~numBit;
+                square9Bit[(row / 3) * 3 + col / 3] &= ~numBit;
+            }
+        }
+        return false;
+    }
+
 
+    boolean isValid(int numBit, int row, int col) {
+        // 左右
+        if ((rowBit[row] & numBit) > 0) return false;
+        // 上下
+        if ((colBit[col] & numBit) > 0) return false;
+        // 9宫格: 快速算出第n个九宫格,编号[0,8] , 编号=(row / 3) * 3 + col / 3
+        if ((square9Bit[(row / 3) * 3 + col / 3] & numBit) > 0) return false;
+        return true;
+    }
+
+}
+
+```
 ### Python
 
 ```python

diff --git a/problems/0236.二叉树的最近公共祖先.md b/problems/0236.二叉树的最近公共祖先.md
@@ -247,7 +247,7 @@ public:
 
 
 ### Java 
-
+递归
 ```Java
 class Solution {
     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
@@ -271,6 +271,47 @@ class Solution {
     }
 }
 
+```
+迭代
+```Java
+class Solution {
+    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+        int max = Integer.MAX_VALUE;
+        Stack<TreeNode> st = new Stack<>();
+        TreeNode cur = root, pre = null;
+        while (cur != null || !st.isEmpty()) {
+            while (cur != null) {
+                st.push(cur);
+                cur = cur.left;
+            }
+            cur = st.pop();
+            if (cur.right == null || cur.right == pre) {
+                // p/q是 中/左 或者 中/右 , 返回中
+                if (cur == p || cur == q) {
+                    if ((cur.left != null && cur.left.val == max) || (cur.right != null && cur.right.val == max)) {
+                        return cur;
+                    }
+                    cur.val = max;
+                }
+                // p/q是 左/右 , 返回中
+                if (cur.left != null && cur.left.val == max && cur.right != null && cur.right.val == max) {
+                    return cur;
+                }
+                // MAX_VALUE 往上传递
+                if ((cur.left != null && cur.left.val == max) || (cur.right != null && cur.right.val == max)) {
+                    cur.val = max;
+                }
+                pre = cur;
+                cur = null;
+            } else {
+                st.push(cur);
+                cur = cur.right;
+            }
+        }
+        return null;
+    }
+}
+
 ```
 
 ### Python