add day-13

Author: libterty

README.md

@@ -13,3 +13,4 @@
     -   Day 10: [Find Town Judge](https://github.com/libterty/leetcode-challenge/blob/master/src/may/day-ten/index.ts) :poop:
     -   Day 11: [Flood Fill](https://github.com/libterty/leetcode-challenge/blob/master/src/may/day-eleven/index.ts) :raising_hand:
     -   Day 12: [Single Element in a Sorted Array](https://github.com/libterty/leetcode-challenge/blob/master/src/may/day-twelve/index.ts) :hear_no_evil:
+    -   Day 13: [Remove K Digits](https://github.com/libterty/leetcode-challenge/blob/master/src/may/day-thirteen/index.ts) :tongue:

src/may/day-thirteen/index.ts

@@ -0,0 +1,28 @@
+/**
+ * @param {string} num
+ * @param {number} k
+ * @return {string}
+ */
+export const removeKdigits = function (num: string, k) {
+    if (num.length === k) return '0';
+    // 堆疊解法
+    let i: number = 0;
+    let stack: string[] = [];
+    while (i < num.length) {
+        while (k > 0 && stack.length && stack[stack.length - 1] > num[i]) {
+            stack.pop();
+            k--;
+        }
+        stack.push(num[i]);
+        i++;
+    }
+    stack = k > 0 ? stack.slice(0, -k) : stack;
+    return stack.join('').replace(/^0+/, '') || '0';
+};
+
+const a = removeKdigits('0354', 1);
+const b = removeKdigits('10200', 1);
+const c = removeKdigits('112', 1);
+console.log('a', a);
+console.log('b', b);
+console.log('c', c);