more recursion problems

Author: beck chen

lessons/recursion/Recursion.drawio

@@ -1 +1 @@
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lessons/recursion/expressionAddOperators.js

@@ -0,0 +1,41 @@
+/**
+ * @param {string} num
+ * @param {number} target
+ * @return {string[]}
+ */
+var addOperators = function (numStr, target) {
+    const results = [];
+
+    function helper(currentIndex, prevNum, totalVal, slate) {
+        if (currentIndex === numStr.length && totalVal === target) {
+            results.push(slate);
+            return
+        }
+
+        const lastIndex = numStr[currentIndex] === '0' ? currentIndex + 1 : numStr.length;
+
+        for (let endIndex = currentIndex; endIndex < numStr.length; endIndex++) {
+            const currentNum = parseInt(numStr.slice(currentIndex, endIndex + 1), 10);
+
+            if (slate === '') {
+                helper(endIndex + 1, currentNum, currentNum, `${currentNum}`);
+            } else {
+                // +
+                helper(endIndex + 1, currentNum, totalVal + currentNum, slate + `+${currentNum}`);
+
+                // -
+                // helper(endIndex + 1, 0 - currentNum, totalVal - currentNum, slate + `-${currentNum}`);
+
+                // *
+                const product = prevNum * currentNum;
+                const curentTotal = (totalVal - prevNum) + product;
+                helper(endIndex + 1, product, curentTotal, slate + `*${currentNum}`);
+            }
+        }
+    }
+
+    helper(0, 0, 0, '');
+    return results;
+};
+
+console.log(addOperators('00000000010', 0));

lessons/recursion/palindromicDecompositionOfAString.js

@@ -0,0 +1,75 @@
+
+/*
+ * Complete the function below.
+ */
+function generate_palindromic_decompositions(s) {
+    const results = [];
+    helper(0, [], s, results);
+    return results;
+}
+
+function helper(startIndex, slate, fullStr, results) {
+    if (startIndex === fullStr.length) {
+        results.push(slate.join('|'));
+        return;
+    }
+
+    for (let endIndex = startIndex; endIndex < fullStr.length; endIndex++) {
+        const currentSubstr = fullStr.substring(startIndex, endIndex + 1);
+        if (isPalindrome(currentSubstr)) {
+            slate.push(currentSubstr);
+            helper(endIndex + 1, slate, fullStr, results);
+            slate.pop();
+        }
+    }
+}
+
+function isPalindrome(str) {
+    let leftIndex = 0, rightIndex = str.length - 1;
+    while (leftIndex < rightIndex) {
+        if (str[leftIndex] !== str[rightIndex]) {
+            return false;
+        }
+        leftIndex++;
+        rightIndex--;
+    }
+
+    return true;
+}
+/*
+ * Complete the function below.
+ */
+function generate_palindromic_decompositions(s) {
+    const results = [];
+    helper(0, [], s, results);
+    return results;
+}
+
+function helper(startIndex, slate, fullStr, results) {
+    if (startIndex === fullStr.length) {
+        results.push(slate.join('|'));
+        return;
+    }
+
+    for (let endIndex = startIndex; endIndex < fullStr.length; endIndex++) {
+        const currentSubstr = fullStr.substring(startIndex, endIndex + 1);
+        if (isPalindrome(currentSubstr)) {
+            slate.push(currentSubstr);
+            helper(endIndex + 1, slate, fullStr, results);
+            slate.pop();
+        }
+    }
+}
+
+function isPalindrome(str) {
+    let leftIndex = 0, rightIndex = str.length - 1;
+    while (leftIndex < rightIndex) {
+        if (str[leftIndex] !== str[rightIndex]) {
+            return false;
+        }
+        leftIndex++;
+        rightIndex--;
+    }
+
+    return true;
+}

lessons/recursion/sudokuSolver.js

@@ -0,0 +1,84 @@
+/**
+ * @param {character[][]} board
+ * @return {void} Do not return anything, modify board in-place instead.
+ */
+
+/**
+ * @param {list_list_int32} board
+ * @return {list_list_int32}
+ */
+function solveSudoku(board) {
+    // Write your code here.
+    const boardSize = board.length;
+    const rowTrackerArr = [],
+        colTrackerArr = [],
+        squareTrackerArr = [];
+
+    for (let ii = 0; ii < boardSize; ii++) {
+        rowTrackerArr[ii] = Array(boardSize + 1).fill(0);
+        colTrackerArr[ii] = Array(boardSize + 1).fill(0);
+        squareTrackerArr[ii] = Array(boardSize + 1).fill(0);
+    }
+
+    for (let rowIndex = 0; rowIndex < boardSize; rowIndex++) {
+        for (let colIndex = 0; colIndex < boardSize; colIndex++) {
+            const num = board[rowIndex][colIndex];
+            if (num === '.') { continue; }
+            rowTrackerArr[rowIndex][num]++;
+            colTrackerArr[colIndex][num]++;
+
+            // fill in squareTrackerArr
+            const squareIndex = getSquareIndex(rowIndex, colIndex);
+            squareTrackerArr[squareIndex][num]++;
+        }
+    }
+
+    function helper(rowIndex, colIndex) {
+        if (rowIndex === boardSize - 1 && colIndex === boardSize) {
+            return true;
+        }
+
+        if (colIndex === boardSize) {
+            return helper(rowIndex + 1, 0);
+        }
+
+        if (board[rowIndex][colIndex] !== '.') {
+            return helper(rowIndex, colIndex + 1);
+        }
+
+        const squareIndex = getSquareIndex(rowIndex, colIndex);
+
+        for (let newNum = 1; newNum <= 9; newNum++) {
+            if (rowTrackerArr[rowIndex][newNum] || colTrackerArr[colIndex][newNum] || squareTrackerArr[squareIndex][newNum]) {
+                continue;
+            }
+
+            // insert number into board and tracker arrays
+            board[rowIndex][colIndex] = newNum.toString();
+            rowTrackerArr[rowIndex][newNum]++;
+            colTrackerArr[colIndex][newNum]++;
+            squareTrackerArr[squareIndex][newNum]++;
+
+            if (helper(rowIndex, colIndex + 1)) {
+                return true;
+            }
+
+            // if helper doesnt return true it means this number isnt promising
+            // so revert it back from the board
+            board[rowIndex][colIndex] = '.';
+            rowTrackerArr[rowIndex][newNum]--;
+            colTrackerArr[colIndex][newNum]--;
+            squareTrackerArr[squareIndex][newNum]--;
+        }
+    }
+
+    helper(0, 0);
+    return board;
+}
+
+function getSquareIndex(rowIndex, colIndex) {
+    return 3 * Math.floor(rowIndex / 3) + Math.floor(colIndex / 3);
+}
+
+const testBoard = [[".", ".", "9", "7", "4", "8", ".", ".", "."], ["7", ".", ".", ".", ".", ".", ".", ".", "."], [".", "2", ".", "1", ".", "9", ".", ".", "."], [".", ".", "7", ".", ".", ".", "2", "4", "."], [".", "6", "4", ".", "1", ".", "5", "9", "."], [".", "9", "8", ".", ".", ".", "3", ".", "."], [".", ".", ".", "8", ".", "3", ".", "2", "."], [".", ".", ".", ".", ".", ".", ".", ".", "6"], [".", ".", ".", "2", "7", "5", "9", ".", "."]];
+console.info(solveSudoku(testBoard)[8]);