diff --git a/problems/0018.四数之和.md b/problems/0018.四数之和.md
index ca33eb57..3fc80bba 100644
--- a/problems/0018.四数之和.md
+++ b/problems/0018.四数之和.md
@@ -136,25 +136,26 @@ class Solution {
         Arrays.sort(nums);
        
         for (int i = 0; i < nums.length; i++) {
-
+		
             // nums[i] > target 直接返回, 剪枝操作
             if (nums[i] > 0 && nums[i] > target) {
                 return result;
             }
-
-            if (i > 0 && nums[i - 1] == nums[i]) {
+		
+            if (i > 0 && nums[i - 1] == nums[i]) {    // 对nums[i]去重
                 continue;
             }
             
             for (int j = i + 1; j < nums.length; j++) {
 
-                if (j > i + 1 && nums[j - 1] == nums[j]) {
+                if (j > i + 1 && nums[j - 1] == nums[j]) {  // 对nums[j]去重
                     continue;
                 }
 
                 int left = j + 1;
                 int right = nums.length - 1;
                 while (right > left) {
+		    // nums[k] + nums[i] + nums[left] + nums[right] > target int会溢出
                     long sum = (long) nums[i] + nums[j] + nums[left] + nums[right];
                     if (sum > target) {
                         right--;
@@ -162,7 +163,7 @@ class Solution {
                         left++;
                     } else {
                         result.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
-                        
+                        // 对nums[left]和nums[right]去重
                         while (right > left && nums[right] == nums[right - 1]) right--;
                         while (right > left && nums[left] == nums[left + 1]) left++;
 
@@ -186,10 +187,10 @@ class Solution:
         nums.sort()
         n = len(nums)
         res = []
-        for i in range(n):
-            if i > 0 and nums[i] == nums[i - 1]: continue
+        for i in range(n):   
+            if i > 0 and nums[i] == nums[i - 1]: continue  # 对nums[i]去重
             for k in range(i+1, n):
-                if k > i + 1 and nums[k] == nums[k-1]: continue
+                if k > i + 1 and nums[k] == nums[k-1]: continue  # 对nums[k]去重
                 p = k + 1
                 q = n - 1
 
@@ -198,6 +199,7 @@ class Solution:
                     elif nums[i] + nums[k] + nums[p] + nums[q] < target: p += 1
                     else:
                         res.append([nums[i], nums[k], nums[p], nums[q]])
+			# 对nums[p]和nums[q]去重
                         while p < q and nums[p] == nums[p + 1]: p += 1
                         while p < q and nums[q] == nums[q - 1]: q -= 1
                         p += 1
@@ -258,12 +260,12 @@ func fourSum(nums []int, target int) [][]int {
 		// if n1 > target { // 不能这样写,因为可能是负数
 		// 	break
 		// }
-		if i > 0 && n1 == nums[i-1] {
+		if i > 0 && n1 == nums[i-1] {  // 对nums[i]去重
 			continue
 		}
 		for j := i + 1; j < len(nums)-2; j++ {
 			n2 := nums[j]
-			if j > i+1 && n2 == nums[j-1] {
+			if j > i+1 && n2 == nums[j-1] {  // 对nums[j]去重
 				continue
 			}
 			l := j + 1
@@ -320,6 +322,8 @@ var fourSum = function(nums, target) {
                 if(sum < target) { l++; continue}
                 if(sum > target) { r--; continue}
                 res.push([nums[i], nums[j], nums[l], nums[r]]);
+		
+		// 对nums[left]和nums[right]去重
                 while(l < r && nums[l] === nums[++l]);
                 while(l < r && nums[r] === nums[--r]);
             }