leetcode: add question 15

Author: yinxin630

15.data

@@ -0,0 +1,4 @@
+[-1, 0, 1, 2, -1, -4]
+[0,0,0,0]
+[-2,0,1,1,2]
+[-2,0,0,2,2]

15.三数之和.js

@@ -0,0 +1,49 @@
+/*
+ * @lc app=leetcode.cn id=15 lang=javascript
+ *
+ * [15] 三数之和
+ * 
+ * 1. 先排序
+ * 2. 选取标杆元素, 左右推进向中间查找. 标杆元素从最左面选取, 后面遇到相同标杆值可以直接跳过
+ * 3. 向中间推进时, 需要跳过相同值
+ *
+ */
+
+// @lc code=start
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var threeSum = function(nums) {
+    nums.sort((a, b) => a - b);
+
+    const result = [];
+    for (let i = 0; i < nums.length - 2; i++) {
+        if (nums[i] === nums[i - 1]) {
+            continue;
+        }
+
+        let a = i + 1;
+        let b = nums.length - 1;
+        while (a < b) {
+            const sum = nums[i] + nums[a] + nums[b];
+            if (sum === 0) {
+                result.push([nums[i], nums[a], nums[b]]);
+            }
+            if (sum <= 0) {
+                do {
+                    a++;
+                } while (nums[a] === nums[a - 1]);
+            }
+            if (sum >= 0) {
+                do {
+                    b--;
+                } while (nums[b] === nums[b + 1]);
+            }
+        }
+    }
+
+    return result;
+};
+// @lc code=end
+